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UNIVERSITY OF CALIFORNIA PUBLICATIONS 
7 1N 


; BOTANY 
Vol.’S, No. 12, pp. 347-428, 10 text figs., plates 49-53 March 6, 1918 


ABSCISSION OF FLOWERS AND FRUITS IN 
THE SOLANACEAE, WITH. SPECIAL 
REFERENCE TO NICOTIANA 


BY 


JOHN -N. KENDALL 


~ UNIVERSITY OF CALIFORNIA PRESS 
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UNIVERSITY OF ILLINOIS 


(2, OW AGRICULTURE LIBRARY 
UNIVERSITY OF CALIFORNIA PUBLICATIONS 
IN 
BOTANY 
Vol. 5, No. 12, pp. 347-428, 10 text figs., plates 49-53 March 6, 1918 


ABSCISSION OF FLOWERS AND FRUITS IN 
THE SOLANACEAE, WITH SPECIAL 
REFERENCE TO NICOTIANA 


BY 
JOHN N: KENDALL 


CONTENTS 


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TUL, MINGLING (GY eee rer ene rea eee Sor eee ear 
IV. Histology and cytology of the pedicel 


1. Histological and cytological condition of the mature pedicel. 363 
2. Development of the separation zone in Nicotiana and Lycoper- 
SACO 2 re ee SO en ee a a 367 
3. Increase in size and development of mechanical tissue in the 
pedicel of Nicotiana and Lycopersicwm .........-..-.--------------------------- 369 
Vem Item TOGESS m0 fercl DS C1SS1O ese ee acne ec wee Senn sew ea cacBsbee aceasta 371 
La 1. General description of the process in several genera —............- 371 
0 2. Method of cell separation —.......--.<----c.----cc-c-eecceceeeeeesee eee 0 ees 376 
i VI. Abscission of the style and corolla .. 383 
7 VII. Time of abscission ..... 385 
cS 1. Reaction time .... . 385 
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a 2. Action of acids on the separation layer of Nicotiana .................- 404 
= oaelma@mctLonb yaamechanicallpemsyuiirys seeese-ceeeecceecee ee saceooearer reser serena enneee 406 
2 4. The ability of certain species to throw off pedicels from which 
= all the floral organs have been removed, as related to the induc- 
tion of abscission by mechanical injury -.. 410 
° TE ASIVILS OTST te ce a Pe ee 411 
£ PROMS OTUGLUS1 0 Leen ere etee ae. neem ee, Wee 2) i A A 415 
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348 University of California Publications in Botany [ Vo. 5 


INTRODUCTION 


Although it is a matter of common observation that many plants 
are capable of detaching portions of the body, the underlying cause 
and the actual mechanism which bring about such separation are 
only slightly understood. The process has often been described as 
one of self-pruning by which the plant rids itself of useless portions 
of its body. Since abscission is sometimes confused with exfoliation, 
it seems desirable here to distinguish definitely between these two 
phenomena. It can be said that, in general, exfoliation is preceded 
by drying and death of the part to be cast off and that actual separa- 
tion of the organ is accomplished by a mechanical break through dry, 
dead tissues. Abscission, on the other hand, is usually not preceded 
by drying and death of the organ concerned and its detachment is 
accomplished by a separation along the plane of the middle lamellae of 
active living cells. 

Abscission may be either axial or lateral. Axial abscission ineludes 
the abscission of portions of stems, shoots, entire flowers or fruits. 
Lateral abscission includes the abscission of leaves, petioles, sepals, 
petals or styles. Considerable attention has been given by investi- 
gators to the abscission of flowers because of the theoretical detriment 
to crops caused by the fall of the flower before the fruit is formed. 

The eause of leaf-fall in deciduous species is connected with peri- 
odie changes in the physiological condition brought about by changes 
in the environment. In the case of some herbaceous plants and ocea- 
sionally in trees, sudden changes in environmental conditions result- 
ing in a loss of physiological equilibrium often cause the throwing 
off of leaves, flowers or even small shoots. In certain species, any- 
thing which tends to loss or completion of function within or peculiar 
to an organ causes the organ to be thrown off. Thus, staminate flow- 
ers are commonly thrown off soon after anthesis and pistilate flowers 
generally fall when fertilization is prevented. Similarly, certain 
species—e.g., Impatiens Sultanti and Mirabilis Jalapa—throw off por- 
tions of their stems which have been rendered useless as a part of the 
conducting system because of injury or removal of distal buds or 
leaves. 

The following definitions of terms, which will be used throughout 
this paper, are made necessary because of a notable lack of uniformity 
in their usage by various investigators who have dealt with abscission. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 349 


1. Abscission is the detaching of an organ by the separation of 
actively living cells at or near its base. 

2. The separation layer (Mohl’s Trennungschichte) is the layer of 
cells the components of which will separate from one another at 
abscission. 

3. The separation cells or absciss cells are the cells that make up 
the separation layer. 

4. The separation zone is the general region through which abscis- 
sion takes place and usually is largely proximal to the separation layer. 

A preliminary account of abscission in F’, species hybrids of Nico- 
tiana has already appeared (Goodspeed and Kendall, 1916). The 
present study represents an amplification of this investigation and its 
extension to other species of the Solanaceae. It is particularly con- 
cerned with the following: (1) the position of the separation layer; 
(2) the origin of the separation layer; (3) the eytology of the separa- 
tion layer; (4) the process of abscission, including (a) a description 
of the appearance of the separation layer in consecutive stages of the 
process and (b) the method of cell separation; (5) the time occupied 
by abscission, including (a) the time between the application of the 
stimulus and fall (reaction period) and (b) the time involved in the 
actual process of cell separation (abscission period) ; (6) experimental 
induetion of abscission. 

Although the investigation reported here is largely a morpholog- 
ical one, the results of the experiments on the method of cell separa- 
tion, the time of abscission and the induction of abscission seem to 
have a distinct physiological significance as well. 


350 University of Califorma Publications in Botany [Vo. 5 


SUMMARY OF THE LITERATURE 


Sinee the literature on abscission is rather voluminous, it seems 
best to present the following discussion under several different head- 
ings corresponding, to a certain extent, with the six main topies of 
interest mentioned in the introduction. The summary below is largely 
confined to the literature on axial abscission, although that on lateral 
abscission is considered in so far as it has a direct bearing on the most 
important aspects of the abscission problem. 


1. HistoLogy oF THE PEDICEL 
a. POSITION OF THE SEPARATION LAYER 


Hoehnel (1880), discussing the fall of catkins in Populus and 
Salix, locates the separation layer at the base of the catkin. The gen- 
eral region at the base of the catkin, in the distal part of which the 
separation layer is located, he calls the ‘‘separation zone.’’ In Salia, 
actual separation occurs in the separation layer, but in Populus it 
occurs in the parenchyma entirely outside the separation layer. 
According to Balls (1911), the separation layer in the cotton flower 
is located at the base of the pedicel. The layer is located by Hannig 
(1913) at the base of the pedicel in Nicotiana Tabacum, N. rustica, 
N. accuminata, N. sylvestris, Datura, and Atropa, and at the tip of 
the pedicel in Nicotiana Langsdorffii, Salvia Aloe, Cuphea, and Gasteria. 
He finds it occurring at the middle of the pedicel in Impatiens Sultani, 
Solanum tuberosum, Lycopersicum, Asparagus, and Begonia. Gort- 
ner and Harris (1914) and Lloyd (1914b), working on the abscission 
of internodes as the result of injury in Impatiens Sultam, locate the 
separation layer at the first node below the injury and just above the 
axillary bud. Occasionally, according to the latter investigators, ab- 
scission may occur at the second or third node beiow the injury and 
in these cases the buds at the first or second nodes seem to be abortive. 

The separation layer, according to Hannig (1913), may occur at 
the base of the complete inflorescence in Impatiens and Oxybaphus. 
According to Lloyd (1914a), the separation layer occurs at the base 
of the pedicel in cotton and at the base of the ripened ovary in grape 
In the abscission of internodes and tendrils in Vitis and 
Ampelopsis, Lloyd (1914a) locates the layer near but not exactly at 
the base of the internode. A peculiar case illustrating the result of 


? 


‘“shelling. 


displacement of the stem on the location of the separation layer is 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 351 


discussed by Lloyd (1914a) for Ampelopsis and Gossypium. In the 
latter, abscission, in the abnormal case, occurred down the internode 
at the base of the pedicel. This is explained as the result of a dis- 
placement during growth by which part of the pedicel becomes united 
to the stem. 

Occasionally, grooves or swellings are noticed at the base of the 
organ being abscissed where they correspond more or less exactly to 
the general position of the separation layer. Examples are given by 
Hannig (1913) for Lycopersicum and Solanum tuberosum and by 
Balls (1911) for Gossypium. Abscission may occasionally occur, 
according to Lloyd (1914a@), above a small bract. According to these 
latter investigators, there is more often no external indication of the 
layer. Frequently, grooves bear no relation to the layer because in 
many cases of this kind (Hannig, 1913, for Brunfelsia) separation 
occurs a short distance distal to the groove. 

From the above brief summary it is evident that in the case of 
axial abscission the separation layer is located at or near the base of 
an internode. Apparent exceptions are reported by Hannig (1913) 
in which it is seemingly located at the middle of an internode. It 
seems probable that a more critical re-examination might reveal the 
fact that even these exceptions accord with the general rule. In these 
eases, for example, the pedicel of the flowers in question might be 
composed of two internodes. 


b. ORIGIN OF THE SEPARATION LAYER 


Kubart (1906) states that the occurrence of the separation layer 
in all tyes of abscission may be explained in one of the three following 
ways: (a) the separation layer is preformed and represents simply 
a portion of the primary meristem which has remained in its original 
active state; (b) it represents a secondary meristem; (c) the primary 
meristem may function directly as a separation layer. The differ- 
ence between a and ¢ is only a difference in time, c being added to 
explain the origin of the separation layer in abscission of very young, 
embryonic tissues. In a, the separation layer is present at the base 
of the organ from the start of its development, but in b this layer has 
to be formed by a secondary meristem before abscission can occur. In 
a, cell divisions are not normally found preceding abscission, but in b 
and c they are. Mohl (1860), working on the fall of the flower in 
Aesculus, Pavia, Lagenaria, Cucumis, and Ricinus, states that the 
separation layer in these forms is of type b. Throughout his entire 


UNIVERSITY OF 
ILLINOIS LIBRARY 


352 University of California Publications in Botany [Vou. 5 


work Mohl gives the general impression that it is necessary for a sep- 
aration layer to be formed from a secondary meristem before abscis- 
sion can occur. Wiesner (1871), working on leaf-fall in general, 
observes that the separation layer is not generally of type b, as Mohl 
believes, but more often of type a. According to Beequerel (1907), 
the separation layer is formed in the pedicel of Nicotiana from a sec- 
ondary meristem (type b). In the cotton flower Balls (1911) finds 
that the separation layer is of type b, but according to Lloyd (1914a 
and 1916b) there is doubt as to this conclusion, since in the ease of 
very young cotton flowers in which abscission occurs very suddenly, 
he finds only rarely that cell divisions do not precede abscission. 
Hannig (1913), for flower-fall in general, states that a separation 
layer of type a is always present but in certain species a secondary 
layer of type b may also be formed, through which separation may or 
may not oceur. Hannig, differing from Beequerel (1907), points out 
that the separation layer in Nicotiana is of type a. Lloyd (1914a) 
and Loewi (1907) indicate that in general a layer of cells through 
which abscission is possible is more often of type a than of type b. 
They believe, however, that the separation layer is not a definite 
morphological structure but represents merely a physiological con- 
dition. 


ce. CYTOLOGY OF THE SEPARATION LAYER 


Mohl (1860) deseribes the separation cells in the flower stalk as 
young, active, small cells which generally contain no starch. He also 
states that in most eases cell divisions are characteristic of the sep- 
aration layer, i.e., that the separation layer is meristematic. Hoehnel 
(1880) finds that cell divisions are characteristic of the proximal por- 
tion of the separation zone in Salix and Populus but in the distal 
portion, where the separation layer is developed, these divisions are 
not so numerous. In some eases he finds separation taking place in 
the parenchyma, entirely outside the ‘‘zone’’ where there had been 
no cell divisions. The separation cells in Nicotiana are described by 
Beequerel (1907) as small, practically undifferentiated cells with 
large nuclei. In Begonia, Fuschia, Mirabilis, and Impatiens Hannig 
(1913) describes the tissue as secondary meristem (type 0) with the 
cells rectangular in shape and arranged in more or less definite rows. 
In contrast to the above observations, he describes the cells as small, 
irregularly arranged and spherical in Salvia, Solanwm nigrum, and 
Nicotiana Tabacum. In Solanum nigrum the separation layer consists 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 353 


of two or three tiers of cells but in VN. Tabacum the layer is made up 
of ten to fifteen tiers. 

Hannig (1913), by means of various microchemical tests, can 
detect no chemical difference between the cell walls of the separation 
layer and those of the cells on either side. Lloyd (1914a), however, 
claims that the cell walls of the separation cells break down more 
quickly when treated with caustic potash than do the walls of normal 
cells. Starch grains are frequently noted by Hannig and Lloyd 
(1916a) as occurring in the separation cells, especially in the abscis- 
sion of internodes by Mirabilis Jalapa. 

An examination of the literature thus makes it evident that there 
has been a great difference noted in the various species in regard to 
the character of the separation cells. The one characteristic of these 
cells, however, to which there is no exception is that they are in an 
actively living condition. 


2. THe Process oF ABSCISSION 


a. METHODS OF ABSCISSION 


It has been found that in practically all cases of abscission the 
detaching of the organ is brought about by the separation of cells 
along the plane of the middle lamella. It is the method noted by 
Mohl (1860), Wiesner (1871), and Kubart (1906), who eall it a pro- 
cess of maceration. Correns (1899) calls it a process of ‘‘schizolysis.’’ 
Correns, however, in the same work describes a new and different 
method of abscission (rhexolysis) which he finds in mosses. In this 
latter method, separation is accomplished by a seemingly passive 
break of tissues irrespective of the position of cell walls. This may 
be the case in the style of cotton (ef. Lloyd, 1914a). This same 
method has been reported by Tison (1900) in the leaf of Aristolochia 
Sipho, although the evidence has been called in question by Lloyd 
and Loewi (1907). Still another type of abscission has been described 
by Hannig (1913) as a result of experiments on Mirabilis and Oxy- 
baphus. In these plants he finds separation being brought about by 
a disorganization and dissolving away of a complete tissue. Lloyd 
(1916a), on the other hand, states that separation in these species is 
accomplished by cell separation and is thus true schizolysis. Hannig 
was doubtless confused in this case by the cell elongations which 
Lloyd observes and by which the membranes surrounding the proto- 
plasts are drawn out exceedingly thin. Loewi (1907), working on 


354 University of California Publications in Botany [ Vou. 5 


several genera, including Cinnamomum and Euonymus, notes and 
figures cell elongations similar to those figured by Lloyd (1916a). 
These cell elongations he finds so frequent and conspicuous that he 
proposes a distinct type of abscission, calling it a ‘‘Schlauchzell 
mechanismus.’’ 

Loewi, on the basis of his studies, seeks to classify the methods of 
cell separation in abscission under six different headings, which per- 
haps would be more appropriately presented under the next subject 
of consideration (the methods of cell separation) ; but since the author 
gave them as distinct methods of abscission they will. be considered 
here. They are: (1) ‘‘round cell’’ mechanism; (2) dissolving of the 
middle lamella; (3) maceration; (4) turgescence; (5) cell elonga- 
tions; (6) ‘‘hard ecell’’ mechanism. They are to be considered merely 
as factors which, singly or in combinations, may enter in as a part of 
the normal process of cell separation. Loewi also claims that by con- 
trolling the temperature, humidity, and various other factors sur- 
rounding the plant he can influence it to such an extent as to change 
its method of cell separation. 


b. METHOD OF CELL SEPARATION 


It has been held by various investigators that the cell separation, 
almost universally connected with abscission, can be caused either by 
(a) chemical alteration and dissolving of the middle lamella or by 
(b) inerease in cell turgor. This whole matter has received consider- 
able attention, although very little direct evidence has been obtained. 
Wiesner (1871 and 1905) states that cell separation is caused by the 
dissolution of the middle lamella and by increased turgor. Kubart 
(1906) and Loewi (1907) agree entirely with Wiesner on this point. 
Strasburger (1913), Tison (1900), Lee (1911), Hannig (1913), and 
Lloyd (1916a and b) believe that cell separation is accomplished by 
the dissolution of the middle lamella. Practically all imvestigators 
have noticed the turgid appearance of the cells after separation, 
although this of course does not constitute evidence that the separa- 
tion is due to increased turgor. Fitting (1911) claims that the sep- 
aration is accomplished, at least in some cases, solely by an increased 
turgor of the separation cells. He bases his claim on the fact that 
abscission is very often too rapid to allow time for the dissolution of 
the middle lamella. He also mentions the fact that the separation 
cells are very often small, spherical cells, the type of cell which would 
respond most readily by an increase in cell turgor. On account of its 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 355 


rapidity and regularity of reaction, Fitting claims that abscission is 
a semi-tropistic phenomenon and suggests the term ‘‘Chorismus’’ to 
designate this type of reaction. 

It has been observed by Hannig and Fitting that the presence of 
various narcotic vapors in the atmosphere around certain species of 
plants causes their flowers or merely the petals to be thrown off. 
Various aspects of this general problem of the reaction of plant tissues 
to such agencies have been investigated. It has been determined by 
various plant physiologists that the presence of narcotic vapors, such 
as illuminating or acetylene gas, in the air around certain plant tissues 
causes the proportion of soluble carbohydrates within their cells to 
increase. This increase in the amount of soluble carbohydrates would 
indicate an increase in cell turgor. The question at once arises, 
whether or not this increase in turgor can effect complete separation 
or maceration of cells without the occurrence of chemical alteration in 
the walls. Richter (1908) resting his case on experimental evidence, 
throws some light on this problem. Various kinds of plant tissues 
which he subjected to acetylene vapors broke in pieces because of the 
maceration and collapse of the living cells within. He finds that in 
the case of the cells of tissues which are commonly rich in starch 
inclusions, such as the fruit of the snowberry and the potato tuber, 
the maceration is most complete. In the potato, for example, 3 to 
5 mm. of material on the surface become completely macerated after 
being subjected to acetylene gas. According to Richter and Grafe 
(1911), the proportion of sugar in starchy seedlings subjected to 
acetylene gas is larger than in seedlings grown under normal condi- 
tions. In seedlings from oily seeds, however, the amount of sugar is 
decreased and the proportion of glycerine and fatty acids increased. 
The conclusion is therefore drawn that the subjection of plant tissues 
to narcotic vapors favors the hydrolysing process in the cells involved. 
The work of these two investigators goes to show that nareotie vapors 
may cause abscission by acting in either of the most important meth- 
ods suggested as responsible for cell separation ; they may increase cell 
turgor on the one hand or favor the hydrolysis of the middle lamella 
on the other. 

Lloyd (1916a) presents evidence of chemical change in the cell 
walls of the separation layer before abscission. These cell walls stain 
in the usual manner with iodine, giving a light brownish color, but 
as abscission commences, they give a faint blue color when stained 
with iodine and washed out with water. Shortly before cell separa- 


356 University of California Publications in Botany [Vou. 5 


tion commences, Bismark brown and Ruthenium red fail to stain the 
primary and secondary cellulose membranes of the separation cells, 
although, when abscission does not occur, the entire cell wall is stained 
in the normal manner. The cells when separating seem, furthermore, 
to be surrounded only by the thin tertiary membranes. Lloyd, in his 
work, figures cells in the process of separation which show the disso- 
lution of the primary and secondary membranes of the cell wall. 

Various interpretations are given to the repeatedly observed 
occurrence of cell divisions preceding and accompanying abscission. 
Mohl (1860) expresses the opinion that cell divisions are generally 
necessary before abscission can oceur. Investigators since his time 
have disproved the universal occurrence of cell divisions because they 
find more and more cases where no cell divisions occur. Lloyd 
(1914a) maintains that cell divisions are not of necessity correlated 
with abscission but are merely evidences of renewed growth and 
wound responses. As evidence he states that cell divisions are some- 
times absent and sometimes present in the same species. He cites 
(19166) the cotton plant as a typical example in which eell divisions 
are present in the abscission of older flowers in which the reaction to 
stimulus is slow. In young flowers and flower buds abscission may 
proceed without cell division. He further notes (1914a) that cell 
divisions sometimes precede and at other times follow abscission in a 
given species. 


c. AGENCIES ACTIVE IN BRINGING ABOUT THE DISSOLUTION 
OF THE MIDDLE LAMELLA 

Very few theories have been proposed to account for the dissolu- 
tion of the middle lamella and practically no evidence of any kind 
has been submitted. Wiesner (1905) claims that im leaf-fall an 
organic acid, produced as a result of lessening of cell activity and 
stagnation of cell contents, acts on the middle lamella. His evidence 
for this statement has to do with obtaining acid reactions with litmus 
from cells at the base of the petiole during abscission. Kubart (1906) 
also obtains acid reactions at the base of the corolla in Nicotiana dur- 
ing abscission and, although agreeing with Wiesner that an organic 
acid probably causes the dissolution of the middle lamella, he also 
admits the possibility that an enzyme plays a part in the process. 
Lloyd (19166) makes the statement that the dissolution of the middle 
lamella is a process of hydrolysis and although making no definite 
statement on the subject appears to take it for granted that an 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 357 


enzyme of some kind is the active factor. Indeed, since all hydrolys- 
ing processes of living cells are now supposed to be due to the action 
of enzymes, there is no reason to suppose that the hydrolysis of the 
middle lamella does not conform to the general rule. For it is known 
that an enzyme, pectosinase, is capable of breaking down the pectose 
of which the middle lamella is composed. However, until more is 
known concerning the nature of this particular enzyme it remains 
impossible to get more definite evidence on this phase of the problem. 


3. ABSCISSION OF THE COROLLA 


Reiche (1885) gives an account of the fall of the corolla in a 
large number of species belonging to about forty-five families of the 
monocotyledons and dicotyledons. He finds that the corolla may be 
thrown off in one of three different ways: (1) by the activity of a 
small-celled separation layer; (2) through deeay; (3) through in- 
crease in size of the ovary, thus tearing off the tissue involved at 
the base of the corolla. In many eases of true abscission—case 1 
above—Reiche finds that the separation layer is preformed and ready 
to function at any moment. This represents a contradiction of 
Mohl’s observations, according to which the fall of the corolla is 
usually due to the action of a separation layer formed shortly before 
fall. According to Reiche, the separation layer is very seldom morpho- 
logically differentiated from the neighboring tissue, but in a few cases 
he describes the separation layer as consisting of a layer of cells 
smaller than the neighboring cells on either side. ; 

Kubart (1906), in his account of abscission of the corolla in sev- 
eral different species, describes and figures the process which takes 
place in Nicotiana. The separation layer in this genus he finds to be 
in no way morphologically differentiated, of indefinite shape, and 
located about 1 mm. above the base of the corolla tube. In this gen- 
eral region a large number of cells separate from one another, all the 
cells in cross-section taking part except the epidermal cells and the 
tracheae. Fitting (1911), in his work on the shedding of petals, de- 
scribes the process of abscission in several genera, paying particular 
attention to Hrodium, Geranium, Linum, Helianthemum, Perlagonium, 
and Verbascum. Separation in these cases takes place through a 
region of small, spherical cells rich in protoplasm. The separation 
layer is not sharply differentiated as compared with the tissues on 
either side but is located in a restricted region at the base of the petal. 


398 Unwersity of California Publications in Botany [Vou. 5 


He finds no e¢ell divisions preceding or accompanying abscission. The 
process in premature abscission he finds differing in no way from 
that in normal abscission after fertilization. These conditions, he 
states, correspond more or less to those which he finds in the pedicel 
during flower-fall. 


4. Time or ABSCISSION 


The time elapsing between anthesis and flower-fall in partially 
sterile F, species hybrids of Nicotiana and between emasculation at 
anthesis and fall in the case of their corresponding parents is dis- 
cussed in a previous paper (Goodspeed and Kendall, 1916). It was 
there stated that the average time is about nineteen days in F, H154, 
seven in F, H179, five in N. Tabacwm var. macrophylla, and thirteen 
in XN. sylvestris. When we turn to the question of the reaction time 
in premature abscission occurring before the normal time as the result 
of sudden changes in external environmental conditions, we find that 
this subject has received only slight attention. According to Lloyd 
(1914@), the cotton ‘‘square’’ falls in one to twenty-two days after 
the weevil lays its eggs, the average time being eight days. In one 
experiment in which the ovary was cut transversely, Lloyd was able 
to cause one hundred per cent of the young bolls to fall in forty-eight 
hours and ninety per cent in twenty-four hours. Larger bolls take a 
longer time to respond to injury than do smaller ones, as a result of 
the- development of the pedicel to a condition in which abscission 
meets greater resistance. Cotton ‘‘squares,’’ he finds, take a longer time 
to respond than young bolls, the former shedding thirty-five to sixty 
per cent in thirty-six hours and the latter forty to seventy per cent in 
forty-eight hours. On the other hand, he obtains no evidence (1916b) 
that the reaction times are any shorter in small buds than in larger 
ones. The reaction times in cases where the injury is performed in the 
evening seem to be shorter by about twelve hours than in cases where 
the injury is performed in the morning. This difference he ascribes 
to the increase in turgidity which takes place during the night and 
which serves to hasten the reaction. Very severe injuries to the ovary, 
he finds, cause fall of young bolls quicker than less severe injuries. 
Injuries which are less severe than those mentioned above and per- 
formed so as to imitate the injury inflicted on the ovary by insect 
larvae caused shedding in three to six days, with most of the fall 
occurring on the fifth day. Summing up his entire results, Lloyd 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 359 


(1916) states that under field conditions the responses to all kinds 
of stimuli conducive to abscission become evident within ten days, 
with the maximum frequency below six days. 

The actual time involved in the process of abscission (abscission 
time) has received even less attention than the problems discussed 
above. Fitting (1911) states that abscission time may occasionally 
be very short, forty-five seconds to five minutes in the petals of Ver- 
bascum and thirty seconds to six minutes in Geranium. Lloyd (1914a 
and 1916) finds abscission after injury of the small cotton-boll taking 
place within four hours, the length of time depending somewhat on 
the age of the boll. In a previous paper (Goodspeed and Kendall, 
1916) a general estimate of the abscission time was given and it was 
stated that normal abscission due to lack of fertilization takes place 
in Nicotiana hybrids in four to eight hours and premature abscission 
in one to four hours. 


5. EXPERIMENTAL INDUCTION OF ABSCISSION 


According to Hannig and Loewi, abscission may be induced in 
two different ways. First by abnormal external conditions (‘‘spon- 
taneous’’ or premature abscission) and second by normal internal 
conditions at the normal time (‘‘automatic’’ or normal abscission). 
We shall consider in the following summary of the literature only 
two aspects of induction of the first type. 


a. INDUCTION BY NARCOTIC VAPORS 


Hannig (1913) reports a comparative study of the behavior of 
cut sprigs of different species of plants when subjected to laboratory 
air and to illuminating gas. He notes the fact that under either of 
the above conditions all the flowers and occasionally a few small 
shoots are abscissed. He finds, however, that not all the species in a 
given family behave similarly in response to these conditions. We 
are particularly interested in the Solanaceae and we may note that 
this family contained more species that detached their flowers in 
illuminating gas than any other of the families investigated by Han- 
mg. According to Fitting (1911), narcotic vapors such as tobacco 
smoke, carbon dioxide, ether, chloroform or illuminating gas fre- 
quently cause premature abscission of the corolla. He notices, how- 
ever, that ammonia or turpentine vapors fail to cause abscission. 
Brown and Escomb (1902) make the statement that Nicotiana, Cu- 
curbita, and Fuchsia shed flowers and buds in air containing only 
0.114 per cent carbon dioxide. 


360 Unwersity of Califorma Publications in Botany [Von. 5 


b. INDUCTION BY MECHANICAL INJURY 


Beequerel (1907), in a brief paper on the effect of wounding 
flowers of Nicotiana, notes that even after fifteen days flowers without 
sepals, anthers, or stigmas do not fall. After the same length of time, 
flowers without corollas or flowers in which the corolla or stamens are 
only half removed, have fallen. He points out that this result is more 
conspicuous in young flowers but did not investigate this point suffi- 
ciently to arrive at any definite conclusions. According to Hannig, 
removal of various organs of flowers frequently causes abscission but 
wounding of the pedicel does not. He concludes, therefore, that in- 
jury itself does not cause abscission but only acts indirectly by inter- 
fering with important physiological processes in the treated tissues. 

According to Lloyd (1914a), shedding of very young cotton-bolls 
can be induced by removal of the styles before pollination, but fall in 
this case can be assigned, as Fitting has shown, to lack of fertilization. 
It appears that in the cotton flower (Lloyd, 1916) there is an inhibi- 
tion period which starts with the opening of the corolla and during 
which premature abscission as the result of sudden stimuli very sel- 
dom occurs. Also, cotton-bolls larger than 30 mm. in diameter are 
very seldom shed under any conditions. Other results obtained by 
Lloyd on the effect of injury on the abscission of cotton flowers are 
discussed above under ‘‘Time of Abscission’’ (page 357). Lloyd 
(19140) also notes the effect of injury on abscission of internodes in 
Impatiens Sultam. Plants of this species, when a cut is made across 
the stem, cast off the remainder of the severed internode. He gives 
results of experiments on the effect of different types of injury, noting 
that some severe injuries do not cause abscission. Gortner and Harris 
(1914) have obtained similar results with the same species. They 
find that when the cut is made across the internode, very close to 
the separation layer, abscission usually oceurs, but occasionally it does 
not. They state, as does Lloyd, that the shape and location of the 
separation layer may vary slightly according to the type of injury. 


c. THE DIRECT OR INDIRECT ACTION OF THE EXTERNAL 
STIMULUS 
In all the above investigations the question naturally arises, 
whether the narcotic vapors and injuries or any stimulus conducive 
to abscission act indirectly through their influence on the physiolog- 
ical condition of the plant or directly, through their action on the 
cells of the separation zone. Most investigators, except Wiesner, ex- 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 361 


press the opinion that atmospheric factors work directly in causing 
““spontaneous’’ abscission, although offering, so far as I can see, no 
evidence for this view. Fitting states that the external influence 
acts directly in most cases, but that the indirect action is apparent in 
forms which must build a separation layer before fall can occur. 
In regard to the action of injury, it seems to be the opinion of most 
investigators (Hannig, Bacquerel, Gortner and Harris) that the 
stimulus acts indirectly by interfering in some way with such 
important physiological processes as transpiration, respiration, or 
assimilation. On the other hand, if abscission is sometimes a semi- 
tropistic phenomenon, as Fitting has suggested, it is evident that 
injury may act directly in causing flower-fall. 


TECHNIQUE 


The results noted below were obtained largely from the examina- 
tion of microscopic preparations made by the paraffin method, 
although this method was supplemented by free-hand sections mounted 
in water. In investigating the condition of the pedicel in some species 
(Datura sp., Petunia sp. and several species of Nicotiana) only free- 
hand sections were examined. For most microchemieal studies fairly 
thick, free-hand sections are preferable. The material for sectioning 
in paraffin was killed and fixed in various concentrations of the 
chromo-acetic series and dehydration and infiltration were, in general, 
carried on very slowly. The free-hand sections were mounted in water 
without killing. 

In cutting longitudinal sections of any kind all the pedicels were 
oriented so that the sections were cut parallel to the main stem of the 
inflorescence, in the plane formed by the pedicel and stem taken 
together. In studying the histology of the pedicel and the cytology 
of the separation layer and in studying the method of cell separation, 
these longitudinal sections were supplemented by cross sections in 
series through the base of the pedicel. It was impossible to cut very 
thin, longitudinal sections in paraffin without crushing or breaking 
the cells; most of these sections therefore were cut from 10p to 15p in 
thickness. For a similar reason, it was found necessary to cut thick 
sections (20p to 25n) of the pedicels of fruits in which mechanical tissue 
had developed. It was possible, however, to cut' excellent paraffin 
sections from 5» to Tu in thickness in cross-section or longitudinally 
through the small cells of the separation zone. Since the cells of the 


362 Unwersity of Califorma Publications in Botany [ Von. 5 


separation zone are very small, not much could be determined in 
regard to the dissolution of cell walls by means of thick, free-hand 
sections. The best results along this line were obtained from the thin 
paraffin sections of the separation zone, although in order to show the 
cell wall in its normal thickness it was necessary to use the free-hand 
sections. As a supplement to these sections, several points of interest 
were brought out by washing off the isolated cells from the end of 
freshly abscissed pedicels and mounting them for microscopic exam- 
ination. 

In most of the work the paraffin sections were stained in safranin 
and Delafield’s haematoxylin. The free-hand sections were generally 
mounted in water and stained in iodine. In special instances other 
stains were used. Thus, in testing for chemical differences in the cell 
walls of the separation cells, several other stains, such as erythrosin, 
eosin, Bismark brown, gentian violet and Ruthenium red were used. 
It was found that for demonstrating the dissolution of cell walls 
aqueous methylene blue was an excellent stain to use. This stain was 
allowed to act overnight and the sections destained slightly in alcohol. 
Methylene blue was also an excellent stain for the isolated cells ob- 
tained as noted above. By fixing these cells to the slide with albumen 
fixative and staining with this stain, the thin membranous wall sur- 
rounding the protoplast can be distinctly seen. 

Various methods, such as subjecting inflorescences to illuminating 
gas and mechanical injury, were used to bring about abscission. The 
best results were obtained in cases where abscission was induced by 
inserting shoots under a bell-jar containing from 1.5 per cent to 
3 per cent illuminating gas. By using illuminating gas in this way 
and by taking sections of the pedicels at intervals it was possible to 
determine just when the first signs of abscission appeared in a certain 
percentage of gas. This time was definitely determined for certain 
species so that it was possible to get material killed and fixed at any 
desired stage in the process of abscission. It was found that the best 
results were obtained by killing and fixing the pedicels at about the 
time when abscission was known to be commencing. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 363 


HISTOLOGY AND CYTOLOGY OF THE PEDICEL 


1. HisronogicaL AND CYTOLOGICAL CONDITIONS OF THE 
Mature PEpIcEL 


a. NICOTIANA 


The vascular system in Nicotiana, as in all the other genera 
examined, is characterized by intraxylary phloem. Nicotiana differs 
slightly from all others in that the xylem seems in cross-section 
to be composed of a continuous ring of radial strands of trachere 
rather than composed of a broken ring of distinct bundles. When a 
branch of the vascular system (fig. 1, a) containing twenty to thirty 
xylem strands is given off to the pedicel, it assumes the shape of a 
erescent in cross-section, with the opening of the crescent on the ven- 
tral side. A short distance distal to the groove which marks the sep- 
aration zone (fig. 1,6), the crescent closes and throughout the 
remainder of the pedicel the vascular system forms a complete eylin- 


Fig. 1. Diagram of pedicel of Nicotiana 


a—vascular system. f—chlorophyllous tissue. 
b—separation zone. g—groove. 

c—pedicel cortex. h—separation layer. 
sc—stem cortex. p—pedicel pith. 


e—epidermis. 


364 University of California Publications in Botany [ Vou. 5 


der. The pith and cortex (fig. 1,p and c) are composed of large 
parenchyma cells which in the cortex are two or three times as long 
as wide, but in the pith are more nearly isodiametric. There is no 
mechanical tissue to be found in the floral pedicel but; as will be noted 
in more detail later, wood fibres are formed as soon as the fruit begins 
to develop. The epidermis of the pedicel (fig. 1, e) is typical but with 
a poorly developed cuticle, especially in the groove (fig. 1, g), where 
the cells are also much reduced longitudinally. Beneath the epidermis 
is a layer of small cells with very large intercellular spaces and an 
abundance of chloroplasts (fig. 1,f/). This tissue stops a short dis- 
tance proximal to the separation zone and does not continue in the 
pedicel. The layer of collenchyma which is commonly found in ecer- 
tain species just beneath this chlorophyl tissue is entirely absent in 
Nicotiana, or at least is very poorly developed. 

Corresponding with the general region of the groove is an ‘area of 
medullary and cortical cells which are smaller than corresponding 
cells on either the proximal or distal side of the groove. This region 
of small cells is homologous with the separation zone (fig. 1,6) and it 
extends across the base of the pedicel. The smallest cells are in the 
center of the region, in a plane with the bottom of the groove, and 
grade in size to the larger cells of the pith and cortex on either side 
(plate 49, fig. 1). The zone of small cells is ten to fifteen tiers of 
cells thick on the dorsal side but is wider on the ventral side, where it 
spreads out into the large area of storage cells found in the axil of 
the pedicel. The separation layer (fig. 1, h) is located five to seven tiers 
of cells distal from the bottom of the groove. Hanning reports this 
layer as occurring at the tip of the pedicel in Nicotiana Langsdor ffi, 
but in all my experiments on two varieties of this species I find separa- 
tion invariably oecurring at the base of the pedicel in the position 
described above. All the species and varieties of Nicotiana examined 
show a structure of the pedicel corresponding with the above descrip- 
tion except that in some varieties, as in those of N. Bigelovii, the sep- 
aration zone is much thinner on the dorsal side. In such eases it is 
also noted that the groove is poorly developed. 

The cells of the separation layer are in no way morphologically 
differentiated from those making up the remainder of the separation 
zone. Indeed, any cell of the zone seems capable of functioning as a 
separation cell. The separation cells are smaller than normal cortical 
cells and spherical in shape except in the vascular bundles, where they 
do not seem to be differentiated in size and are elongated parallel to 
the longitudinal axis of the pedicel. The cell walls are slightly thicker 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 365 


than the walls of normal cortical cells, especially at the corners, thus 
giving the tissue a somewhat collenchymatous appearance. The small- 
est cells more proximal show this collenchymatous nature more strik- 
ingly than do the others. No difference in chemical composition could 
be detected, by means of microchemical tests using caustic potash, 
sulfuric acid, nitric acid, and various stains, between the cell walls of 
the separation cells and walls of other cortical cells. Other tests, how- 
ever, indicated a difference in the nature of the cell contents in the 
two types of cells. Iodine frequently indicates the presence of starch 
in these cells and also colors the protoplasts a darker brown than in 
normal cells, showing that the separation cells are rich in protoplasm. 
The amount of starch in the cells, however, was found to be extremely 
variable, ranging from a total absence of starch to an abundance of 
it. Iodine green imparts to the protoplast of the separation cells a 
deep blue color in contrast with other cortical cells, which are not 
colored by this stain. The blue reaction is most prominent where the 
separation layer crosses the phloem. Other cells which react in the 
same way to this stain are the sieve tubes and companion cells and 
the storage cells in the axil of the pedicel. 


b. LYCOPERSICUM 


Conditions in Lycopersicum differ in certain respects from those 
existing in Nicotiana. In the former the separation zone (fig. 2, a) 
seems to be located at the middle of the pedicel 
and is marked externally by a swelling, as well 
as by the groove of the type already noted as 
characteristic of the pedicel of Nicotiana. This 
groove in the tomato is very deep (plate 53, 
fig. 1), reaching fully half the depth of the 
cortex, and is, furthermore, of about the same 
depth all the way round, differing in this 
respect from Nicotiana, where the groove is 
absent or poorly developed on the ventral 
side. The vascular system in Lycopersicum 


Fig. 2. Diagram of pedicel __, : 5 ang 2 
of Dacsperstoum (fig. 2, b), in contrast with the condition in 


J SREVEE EEN ACE. Nicotiana, is composed of scattered bundles 


b—vascular system. : 

c—epidermis. of xylem which in this case do not form a 
d—separation layer. P - 
copithe y crescent proximal to the groove but are in the 
Jove eeuonul bearing form of a complete cylinder throughout the 


g—collenchyma entire pedicel. Beneath the epidermis (fig. 


366 University of California Publications in Botany [ Vou. 5 


2, c) is the chlorophyl-bearing region of the cortex (fig. 2, f), such 
as occurs in Nicotiana, but in this case the tissue continues in the 
pedicel distal to the groove. Beneath this chlorophyl-bearing tissue 
is a layer of well-developed collenchyma (fig. 2, g) which however 
does not continue in the pedicel distal to the groove. The separation 
layer (fig. 2, d) consists of three to six tiers of cells and is located 
in a plane with the groove, differing in this respect from Nicotiana, 
where it is located a short distance distal to the groove. Correspond- 
ing to the condition in Nicotiana, the chief characteristic of the separa- 
tion cells is their small size, spherical outline and active physiological 
condition. 


c. OTHER GENERA OF THE SOLANACEAE 


The condition of the pedicel, so far as the histology of the separa- 
tion zone is concerned, was examined in several other species, a list 
of which is given below: 


Solanum jasminioides Cestrum fasciculatum 
Solanum tuberosum Iochroma tuberosa 
Solanum verbascifolium Datura sanguineum 
Solanum umbelliferum Salpichrora rhomboidea 
Solanum nigrum Petunia hybrida 
Solanum marginatum Salpiglossis sinuata 


Lycium australis 


The general condition of the pedicel of Datwra sanguineum and 
Petunia hybrida is worth describing in some detail. The tissues of 
plants of D. sanguineum are more or less herbaceous in nature, large- 
celled and somewhat succulent throughout. The chlorophyl-bearing 
tissue which, in striking contrast with the condition in Nicotiana and 
Lycopersicum (figs. 1 and 2), is continuous over the separation zone, 
is composed of two rows of small, spherical cells just beneath the 
epidermis. Except for a layer of collenchyma, whose much elongated 
cells extend the entire length of the pedicel and thus continue the col- 
lenchyma through the separation layer, the cortex and pith are com- 
posed of more or less isodiametric, thin-walled cells. Floral abscission 
is as common in this species as it is in Nicotiana. The flowers are 
very large and furnish excellent material for a study of the cytology 
of abscission. Unfortunately not a sufficient number of flowers could 
be obtained to make possible any detailed study of this genus. It was 
noticed, however, that there is no region of small cells at the base of 
the pedicel within which separation occurs and that the separation 
cells are identical in size and shape with those on either side among 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 367 


which separation does not occur. The separation layer here is located 
about 8 mm. distal to the base of the pedicel, with absolutely no ex- 
ternal indication of its position. Muicrochemical tests, which in Nico- 
tiana gave different reactions in the case of the separation zone and in 
the case of normal cortical cells, here fail to show any corresponding 
condition of differentiation. 

Abscission has never been found to oeeur in Petunia or Salpiglossis, 
so that it is of interest to examine the histological condition of the 
base of the pedicel in these two species. They are practically identical 
with regard to the structure of the pedicel, so that the description 
given below can be taken as applying to both genera. The cortical 
cells of the pedicel pass into those of the stem without any groove or 
small-celled region. On the ventral side, however, is the region of 
small cells in the axis of the pedicel, which is more or less common to 
all flowers. The tissues of Petunia are not so soft and succulent as 
those of Datura, Nicotiana, and Lycopersicum. They tend rather to 
be dry and tough. The cells in the cortex and pith are also not so 
nearly isodiametric as in Datura, but are much elongated in a direc- 
tion parallel with the long axis of the pedicel. 

The condition in the other species mentioned above will be given 
only a general description. Abscission occurs in all the other species 
except Salpichrora and Lyciwm which, however, do not differ, in 
respect to the histology of the base of the pedicel, from any of the 
others. Solanum tuberosum resembles Lycopersicum. All the other 
species are similar in regard to the structure of the separation zone. 
There is in every case a general region of small cells extending 
across the base of the pedicel where the separation layer occurs. 


3. DEVELOPMENT OF THE SEPARATION ZONE IN Lycopersicum 
AND Nicotiana 


a. LYCOPERSICUM 


The development of the separation zone could be followed better 
in Lycopersicum than in Nicotiana because in the former the zone is 
not so close to the main axis of inflorescence. The problem here 
resolves itself into an effort to determine, by means of longitudinal 
sections of very young pedicels, how early in the development of the 
flower the groove and the differentiation in cell size of the separation 
cells appear. It was found that the development of the separation zone 
indicates the method by which the groove and differentiation in cell 


368 University of California Publications in Botany [ Vou. 5 


size originate. The groove is fairly well developed (fig. 5) in young 
buds whose corolla is only 3mm. in length, but is not so deep as in 
older buds. The cells of the separation zone at this stage are smaller 
than cells on either side, but the difference is not so prominent as in 
older flowers. In very small buds whose corolla is only 1mm. in 
leneth or whose calyx is only 2 mm. long, the groove is just beginning 
to appear (fig. 4). In buds below this size (fig. 3) no groove or 
differentiation in cell size can be detected. Abscission can occur in 
these early stages, before the groove or differentiation in the size of the 
separation cells has appeared, as well as at any other stage. In these 
early stages the radial diameter of the cortex is much less, as com- 
pared with that of the pith, than in older flowers. It is evident, 
therefore, that the cells of the separation zone are small because they 
retain their original small size while the rest of the cortical cells 
increase in size. The fact that the groove is formed makes it probable 
that there have been few cell division, or none, in the separation zone 
of the cortex during the development of the bud. It was observed, 
however, that the cells of the separation zone in the pith retain their 
meristematic nature for a considerable period during the development 


Fig. 3 Fig. 4 


+ 


Fig. 5 


a—separation zone 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 369 


of the flower. They are at this time rectangular in shape, elongated 
perpendicularly to the long diameter of the pedicel, and arranged in 
longitudinal rows. In later stages, however, when the flower is at 
anthesis, or the fruit is forming, these cells have rounded up and 
become irregularly arranged, thus leaving rather large intercellular 
spaces. 

b. NICOTIANA 


The separation zone develops in Nicotiana much as it does in 
Lycopersicum. It was observed in very young buds—calyx 2 or 3 mm. 
or shorter—that no groove was present. In buds larger than these, 
the groove and small size of the separation cell is apparent, appearing 
first on the dorsal side of the pedicel. It is evident that in both these 
genera the groove and area of small cells are explained in the same 
way, i.e., by the fact that the normal cortical cells increase in size 
faster than do the cells of the separation zone. Since in both genera 
abscission can occur even before differentiation of any kind appears at 
the base of the pedicel, it is evident that the groove and small-celled 
region do not necessarily bear any relation to abscission. This state- 
ment is borne out by the fact that in Datura neither the groove nor 
the area of small cells is present and in Nicotiana separation occurs a 
short distance distal to the groove. 


c. CONCLUSIONS FROM THE STUDY OF THE DEVELOPMENT OF 
THE SEPARATION ZONE 

Tn view of the above discussion it is clear that the separation layer 
in Lycopersicum, Nicotiana, Datura, and probably in the other genera 
noted, originates according to the first method, a, proposed by Kubart 
(cf. page 350). That is to say, the separation layer represents merely 
a portion of the primary meristem which retains its original physi- 
ological capacities. 


4. INCREASE IN SIZE AND DEVELOPMENT OF MECHANICAL TISSUES IN 
THE PEpIcEL oF Nicotiana and Lycopersicum 


There is a marked increase in the size of the pedicel in both 
Nicotiana and Lycopersicum during the development of the fruit. It 
was found that during this development the diameter of the pith 
remains about the same, the actual increase in size being almost 
entirely confined to the cortex (ef. figs, 3, 4, and 5). This increase in 
the diameter of the cortex in the pedicel of Nicotiana is due, in the 
first place, to an increase in the size of the original cortical cells, 


370 Umversity of California Publications in Botany [Vou. 5 


which in average cases measured about 20u in diameter in the flower 
and about 40 in the fruit. In the second place, it is due to four or 
five divisions of the cambium layer. This second factor in the increase 
in size of the pedicel becomes evident when a count is made of the cells 
between the phloem and trachex, the result giving approximately six 
cells in the flower and eleven in the fruit. 

The increase in size of the pedicel of Lycopersicum, which is much 
more prominent than the. increase in Nicotiana, can be explained in 
the same manner. In the former the increase in size, which in this 
case takes place almost entirely distal to the groove, may proceed to 
such an extent that the diameter of the pedicel of the fruit is two or 
three times that of the flower at anthesis. A measurement of the 
cortical cells in cross-section gave on the average 10 in the flower and 
284 in the fruit. In this case only two or three divisions of the 
cambium occur; the cells resulting immediately show lhenification. 

The next subject of consideration is the development of mechanical 
tissue in the pedicel of Nicotiana and its relation to abscission. It will 
be remembered that there was no mechanical tissue noted in the 
pedicels of buds and flowers. Parallel with the development of the 
fruit, however, a continuous ring of mechanical tissue appears in the 
xylem of the pedicel. This mechanical tissue is evidently the result 
of a gradual lignification of the cells of the cambium and the outside 
portion of the xylem parenchyma. There is thus formed a continuous 
sheath of what may best be called wood-fibre tissue, in the form of a 
cylinder just outside the tracheal elements. These mechanical elements 
first appear in the tissues of the pedicel five or six days after anthesis, 
but since the lignification in these more distal tissues is merely the 
result of the spreading upwards of the lignification in the older parts 
of the plant, this period depends somewhat on the position of the 
flower on the inflorescence. It was noticed in Nicotiana that the 
wood-fibre tissue develops on both sides of the separation zone before 
appearing in the latter, but in time it becomes continuous through 
the separation layer. By a lignification of the cells between the two 
points of the crescent of wood in the separation zone, there is also 
a slight tendency to close this crescent on the ventral side. 

Since abscission has not been observed to oceur in lignified cells, 
the question at once arises whether the tough sheath of lignified cells 
which continues through the separation layer could hold the fruit on 
the stem even after actual abscission had occurred. Upon looking 
over any large number of plants in the field it will at once be evident 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 371 


that such a condition of affairs very often exists. It will be found in 
many cases, especially on older plants, that although abscission has 
oceurred in the cortex, as evidenced by the presence af a white, 
powdery substance at the base of the pedicel, the capsule is yet firm 
on the stem. Indeed, in certain hybrid tobaccos it is common to find 
most of the capsules in this abscissed condition. The fruit is supported 
in these cases by the tough mechanical elements of the wood, which 
also prevent the breaking of the trachee and protect the intraxylary 
phloem. In the pith the tissues may be in a somewhat abscissed con- 
dition, but since there is no way for these cells to escape through the 
sheath of wood they remain for some time in position before finally 
collapsing. 

The development of mechanical tissues takes place in Lycopersicum 
in much the same manner as in Nicotiana but with the distinct differ- 
ence that in the former the wood-fibre tissue does not become con- 
tinuous through the separation layer. That is to say, in the tomato 
a break is left in the mechanical tissue in a plane with the bottom of 
the groove. It is evident here that abscission would cause fall of the 
fruit in any stage of its development, although in this case it happens 
that abscission very rarely occurs after two or three days past anthesis. 
A condition resembling this one in the tomato was observed in other 
berry-forming species of the Solanaceae such as Cestrum fasciculatum 
and Solanum verbascifolium, which often drop their immature fruits 
by abscission. Abscission, however, very seldom occurs in mature 
berries of these species, the fruit generally falling away from the 
receptacle above the calyx. 


THE PROCESS OF ABSCISSION 


1. GENERAL DESCRIPTION OF THE PROCESS IN SEVERAL GENERA 


a. NICOTIANA 

The process of abscission in all the species of Nicotiana investi- 
gated conforms to the usual type involving separation and isolation of 
cells. Further details of the process were briefly discussed in a pre- 
liminary paper (Goodspeed and Kendall, 1916) for certain F', species 
hybrids of Nicotiana. It was there noted that cell separation starts in 
the dorsal side of the pedicel, in the cortex a short distance distal to 
the groove (pl. 49, fig. 1) and spreads from this point around to the 
ventral side. The first external indication seems to be a bulging of 


372 University of California Publications in Botany [ Vou. 5 


the epidermis (pl. 49, fig. 2) over the tissue in which the process is 
taking place. Simultaneously with the start of abscission in the 
cortex, the process apparently originates independently in the pith 
(pl. 50, fig. 1). It was further noted that the number of cells con- 
cerned in the process, as a general rule, is greater in the hybrids than 
in their parents and also that this is true of ‘‘automatic’’ as compared 
* abscission. Just beneath the epidermis the cells 
involved in separation were reported as being from five to ten tiers 


with ‘‘spontaneous’ 


thick, but as the process approached the vascular tissue the separation 
layer was evidently reduced in thickness to not over one or two tiers 
of cells (pl. 52, fig. 1). In the pith a more or less spherical mass of 
cells is involved (pl. 50, fig. 1). When the separation is completed 
the flower may remain in position for some time, until the epidermis 
and tracheal elements are broken by some mechanical agency. 

The exposed separation surface of the pedicel was stated to be 
convex in outline and slightly notched at the tip. Upon closer exam- 
ination the surface itself was seen to be composed of the protruding, 
rounded ends of cells with here and there completely isolated cells and 
broken ends of spiral trachew. These isolated cells are apparently 
normal and do not markedly differ in form, size, or in the nature of 
their cell inclusions from the same cells before separation. The 
exposed surface of the attached portion of the pedicel is similar in 
appearance to that of the detached portion, but is more or less flat 
in outline. After separation the cells on this surface collapse and 
probably act as a protective layer. 

Following the observations recorded above, which had to do largely 
with flower-fall in the F, species hybrids, a number of species have 
been investigated in an effort to determine whether or not their mode 
of abscission differs from that already described. 

It may be noted at the start that no marked exceptions were found 
to the previously described condition, although at least two stages in 
the process of abscission have been found to be subject to considerable 
variation. The first of these stages has to do with the place of origin 
of the abscission process itself. An independent origin in the pith 
has been demonstrated to occur in a large number of species and 
occasionally it was found that the first evidences of abscission could 
be detected here before any similar evidences appeared in the cortex. 
Again, it was found in most species that cell separation starts first in 
the ventral cortex although other places of origin were found in 
several cases. Thus, in Nicotiana Tabacum ‘‘Maryland’’ and F, H36, 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 373 


for example, the process originates on the ventral side and may even 
spread through the large area of storage cells in the axil of the pedicel 
before reaching the dorsal side. The distance distal from the bottom 
of the groove at which separation appears is also subject to variation. 
This variation, however, is not typical of certain species, since it may 
oceur at different times in the same species, evidently as a result of 
an abnormal stimulation to abscission. 

The second part of the process subject to variation has to do with 
the amount of tissue that may be concerned in actual cell separation. 
Abscission first becomes complete in a narrow plane between two or 
three tiers of cells across the pedicel and the flower can be easily 
shaken off at that time. If, however, the flower remains on the stem, 
and is kept turgid by the water rising in the unbroken tracher, cell 
separation spreads more and more widely through the tissues of the 
pedicel, especially in the pith and cortex. It is the extent to which this 
spreading normally proceeds that varies in the different species. When 
the process has spread to a considerable extent, a white ring formed 
by the isolated masses of cells can be seen with the naked eye at the 
base of the pedicel and a casual inspection indicates that the amount 
of this white substance varies in the different species. In most 
hybrids, except F, H179, there is more spreading in normal abscission 
than in pure species. In Nicotiana quadrivalvis, N. Bigelovii, and 
other similar species in which abscission very seldom occurs, no spread- 
ing takes place. Spreading, however, occurs to a remarkable extent in 
N. Tabacum ‘‘Maryland.’’ 


b. LYCOPERSICUM 


We may say that, in general, abscission in Lycopersicum corre- 
sponds to that in Nicotiana and that the main points of distinction 
between the two arise only from the original differences in the separ- 
ation zones (ef. page 364). In addition, attention must be called to 
the fact that quite frequently, in individual plants of the tomato, no 
true abscission occurs in normal flower-fall. In these cases the flower 
seems to be detached from the plant by a process which compares 
closely with that called exfoliation. There is no active cell separation 
and the flower simply wilts and dries back to the groove, where it 
hangs until broken off by some mechanical agency. The first indica- 
tion of the process is the loss of chlorophyl in the pedicel, which 
gradually turns yellow, commencing at the tip and spreading proximal 
to the separation zone. It is possible that most of the flower-fall 


374 Unwersity of California Publications in Botany [ Vou. 5 


noticed by agriculturists is of this type. Quite often, however, true 
abscission and this second type of flower-fall may both be found 
operative in the same plant or even in the same flower. ‘‘Spon- 
taneous’’ flower-fall in the tomato is, of course, of the true abscission 
type. 

Corresponding with the condition in Nicotiana, true abscission in 
Lycopersicum is seen to originate frequently in the pith. At any rate, 
the process goes on here independently of that in the cortex, since the 
final break is through the trachexw and epidermis. Furthermore, separ- 
ation takes place in a plane with the bottom of the groove (pl. 53, fig. 
2) whereas, in Nicotiana, it takes place a short distance distal to the 
groove. Separation may at first take place between only two tiers of 
cells (pl. 53, fig. 2), but in time the process may spread until three 
or four tiers become involved in separation. However, there is no 
spreading of the process to a large number of cells, as is frequently 
seen in Nicotiana, so that one very seldom finds the white powdery 
substance at the point of separation. Also in contrast with the con- 
dition in Nicotiana, there is, as abscission progresses, no bulging of 
the epidermis which instead soon breaks in the bottom of the groove. 
Separation in the tomato takes place in such a way as to give the 
exposed separation surfaces the same general shape after abscission as 
in Nicotiana, that of the detached portion of the pedicel bemg convex 
and that of the remaining portion slightly concave. 


c. DATURA 


Conditions in Datura differ strikingly from those in the two 
species described above. This would be expected when one considers 
the great differences in the structure of the separation zones (cf. 
page 365). In Datura there is the usual chlorophyl-bearing tissue, 
which consists of two rows of small, perfectly isodiametric cells with 
large intercellular spaces, just beneath the epidermis. It will be 
remembered from the description on page 365 that this tissue in Datura 
continues the entire length of the pedicel and therefore, in contrast 
with the condition in Nicotiana and Lycopersicum, extends through 
the separation zone. The first sign of abscission is the maceration of 
this tissue as indicated by the appearance of a white color under the 
epidermis. The latter may as a result become detached from the tissues 
of the cortex for a distance of 2 em. or more along the base of the 
pedicel. This is soon followed by a break over the separation layer 
and a curling back of the epidermis on either side, with most of the 
chlorophyl-bearing cortical tissues still attached to its inner surface. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 375 


After the break in the epidermis separation continues in the layer 
of collenchyma just beneath. The cells of the collenchyma layer, 
which are much elongated parallel to the long axis of the pedicel (five 
to eight times as long as wide), separate for a distance of about 0.3 
mm. up and down the pedicel, involving only a few tiers of cells. It 
is evident that the cells of this tissue separate without difficulty, 
although not by any means as freely as the small spherical cells de- 
seribed above. The large, isodiametric, parenchyma cells of the cortex 
separate for a distance of 2 or 3 mm., involving many tiers of cells. 
The cells of the starch sheath, which are small and spherical, separate 
for a distance of 1 em. or more, thus causing a longitudinal cavity to 
be formed just outside of the vascular bundles. In the latter, separa- 
tion involves only two or three tiers of cells. Separation originates 
and continues in the pith independent of the process in the cortex, 
but involves about the same number of cells as in the parenchyma of 
the latter tissue. When separation has thus become complete, the 
weight of the flower is very often sufficient to break the trachee and 
eause the flower to fall to the ground. 

Several important facts are brought out by this examination of 
abscission in Datura. In the first place, it shows that floral abscission 
ean take place without any structure which might possibly be inter- 
preted as a morphologically differentiated separation layer. In the 
second place, it indicates that cell separation is possible in several dif- 
ferent types of living cells. It also shows that separation takes place 
more readily in small cells than in large ones and more readily in 
isodiametric cells than in elongated ones. The theory that the separa- 
tion layer is not a morphologically differentiated structure, but repre- 
sents a physiological condition (Lloyd and Loewi), could certainly be 
well applied in this case. 


d. OTHER GENERA 


The process of abscission in the other species listed on page 365 is 
essentially the same throughout. No indications were noted of cell 
divisions or elongations accompanying abscission. Separation is 
brought about by means of a separation of small and active cells 
located in the general region at the base of the pedicel. In all these 
forms the separation surface of the pedicel is convex in outline, so 
that the separation layer must lie in more or less of a crescent in the 
stem at the base of the pedicel. The main difference between these 
forms and the three that have been described in detail above is found 


376 Uniwersity of California Publications in Botany [ Vou. 5 


in the fact that in the former, with the exception of Solanum tuber- 
osum, separation occurs in the stem at the very base of the pedicel, 
whereas in the latter three it occurs through the pedicel a varying 
distance from the base. 


2. MrerHop or CELL SEPARATION 
a. GENERAL REMARKS 


It will be remembered that two theories have been proposed to 
account for the cell separation that is responsible for abscission. 
First, it is conceivable that cell separation may be caused by an 
increase in cell turgor, which causes the cells to round up and pull 
apart without any change taking place in the chemical nature of the 
middle lamella. Second, cell separation may be caused by a chemical 
dissolution of the middle lamella with or without an increase in cell 
turgor. The main difference between the two theories is that the 
second, in contrast with the first, maintains that chemical alteration 
of the middle lamella is always necessary before abscission can occur. 
The first theory gains support from the work of Fitting and the second 
from the work of Hannig, Lee, Strasburger, and Lloyd. Wiesner, 
Kubart, and Loewi believe that cell separation takes place by the 
action of both factors but that either factor may at times be the more 
important. 


b. CYTOLOGICAL CHANGES ACCOMPANYING ABSCISSION 


It was stated in a preliminary discussion (Goodspeed and Ken- 
dall, 1916) first, that no indication of cell divisions or elongations 
were observed accompanying abscission, and, second, that no evidence 
of the dissolution of the middle lamella had at that time been obtained. 
The first statement has been corroborated in that, during all the later 
experiments, no divisions or elongations have been observed in any of 
the described species. The dissolution of the primary cell membrane, 
however, because of more exact knowledge of the proper time to take 
sections and of more successful staining methods, has been fairly well 
established. 

The main problem here was to determine by the use of various 
stains whether or not the primary and secondary cell membranes of 
the separation cells stain differently in the early stages of abscission 
than under normal conditions. This was a point which was found 
very difficult to determine, principally because of the fact that the 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 377 


separation cells are, comparatively speaking, very small, but also be 
cause of the fact that the walls of these cells fail to show any strati- 
fication. 

Jodine, Delafield’s haematoxylin, Ruthenium red, Bismark brown, 
methylene blue, erythrosin, and eosin were used with little success in 
most cases. By using iodine, however, just as abscission is known to be 
commencing, a white streak may be seen across the section in the region 
of the separation layer. Upon careful examination it was decided 
that this white streak was due to the failure of most of the cell walls 
in the separation layer to take the stain. Although it is probable 
that with more careful examination the other stains mentioned above 
would give similar results, it was found that methylene blue was the 
only stain with which anything definite could be established. If a 
thin longitudinal section cut in paraffin as abscission is known to be 
starting, and stained in methylene blue, is examined (cf. page 361), it 
will be found that the walls of those cells in which separation is about 
to oceur have remained almost entirely unstained. The protoplasts in 
these cases seem to be surrounded only by the thin tertiary mem- 
branes, between which is a streak of colorless material of varying 
width (pl. 51). Cell walls where separation is not expected to occur, 

‘however, stain a dark blue throughout in the normal manner. 

An examination of freshly isolated cells washed off from the end 
of an abscissed pedicel shows that these cells are still turgid and 
active. It was impossible to determine whether these cells had in- 
creased in size, as compared with the size of similar cells before abscis- 
sion, but it is evident that the inerease, if any, had not been very 
great. The cells still contain their large nuclei, and occasional starch 
grains, and show after isolation no signs of degeneration even after 
several hours in water. In addition, these isolated cells appear to have 
retained their original shape. In the collenchyma of Datura the cells 
are from five to eight times as long as wide, and yet these cells retain 
their original shape when isolated, as a result of the dissolution of 
the middle lamellae. This isolation has evidently not been complete, 
since large masses of cells are seen still attached to each other. It is 
noticed that in all cases the protoplast is surrounded by an extremely 
thin membranous wall (pl. 52, fig. 3). It is also frequently noticed 
that the protoplast seems drawn away from the cell wall as if plas- 
molysis had oceurred. It is possible that this appearance may be due 
simply to the gathering together of granules and the denser portion 
of the protoplasm in the center of the cell. 


378 Unwersity of Califorma Publications in Botany [Vou. 5 


c. EXPERIMENTAL EVIDENCE FOR THE DISSOLUTION OF 
THE MIDDLE LAMELLA 


It is supposed that the middle lamella, or primary cell membrane, 
is largely composed of calcium pectate, a calcium salt of pectic acid 
which has been given the general name pectose. The secondary cell 
membranes probably contain a larger proportion of cellulose with 
the pectose than is present in the primary membranes. This pectose, 
which is of course insoluble in water, is disorganized by a process of 
hydrolysis to form pectin. The pectin, which is a colorless mucilagi- 
nous substance, is readily soluble in water but is precipitated along 
with the proteids and enzymes of the protoplast by the addition of 
alcohol. Thus, if a water extract is made from separation zones dur- 
ing the first stages of abscission, one would expect to get a solution of 
several substances, among which would be the pectin produced by the 
dissolution of the pectose in the primary cell membranes. It might 
be expected that the amount of precipitate obtained from this extract 
with alcohol would be greater, provided the amount of other sub- 
stances remained the same, than the amount of precipitate obtained 
in a similar manner from separation zones in which there had been 
no abscission and in which no pectin had been formed. Whether or 
not the inerease in the amount of precipitate is due to the added 
pectin cannot of course be proven without actual chemical analysis, 
and such an analysis would be difficult because of the very small sam- 
ples of material obtainable. However this may be, any difference in 
the amount of precipitate would be of interest. 

This experiment and the two which follow are, as far as I have 
been able to determine, the first of their kind. Apart from this fact, 
their chief value probably lies in the fact that they suggest a line of 
investigation which, if carried on in more detail and with better 
facilities, will undoubtedly lead to important conclusions. These 
experiments were, however, carried on with as much care as possible - 
and since the results of duplicate tests are in agreement, they give, as 
far as they go, dependable results. 

After several experiments indicating the results given below, the 
following test experiment was performed: 

Experiment 1.—Two water extracts of equal concentration were 
made from the lots of material. Lot A contained 200 small pieces of 
the pedicel in which the separation zone was located and in which 
abscission had started. Lot B contained an equal weight of a similar 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 379 


number of pedicels in which no abscission had started. The extracts 
were made up to 10 ce. and the precipitate obtained with 60 ce. of 
95 per cent alcohol. The precipitate weighed in the two lots: 


NGS Sie a ol 7 Ree Se ae EE a ee 996 me. 
IE ewes Skee ee Oe ee A a 903 mg. 


One of the preliminary experiments performed with a weaker 
alcohol gave results which may or may not be of considerable impor- 
tance. In this experiment a light, almost invisible precipitate formed 
in A and no precipitate in B. Whether or not the pectins precipitate 
in lower percentages of alcohol more readily than the other substances 
I have been unable to determine. At any rate, the precipitate in this 
ease felt slimy and mucilaginous to the touch and might well have 
been the precipitated pectin approximately pure. 


d. EVIDENCE FOR INCREASE IN TURGOR 


It was stated along with other conclusions in the preliminary paper 
(Goodspeed and Kendall, 1916) that from the evidence at that time 
available it was probable that cell separation is caused merely by an 
increase in cell turgor, and throughout this later work it has been 
clear that increased turgor is present during abscission. In view of 
the evidence given above, however, it would seem that turgor can 
play only a secondary role, although the occurrence of imerease in 
turgor must not be ignored. ; 

The bulging of the epidermis frequently noted as accompanying 
abscission is evidence of increased internal pressure. In the pith the 
cells next to those which are separating are in a collapsed condition 
due to the pressure of the expanding separating cells. By various 
experiments it can be shown that humid conditions favor and severe 
drought prevents abscission. Richter and others have shown that 
nareotic vapors which. cause abscission also cause increased turgor by 
increasing the proportion of sugar in starch-containing cells. This 
increase in cell turgor becomes so great as to cause complete macera- 
tion in certain types of tissues. The frequent presence of starch 
grains in the separation layer of Nicotiana, part of which are prob- 
ably converted into sugar as a result of subjection to illuminating 
gas, indicates that there is probably an increase of turgor during 
abscission, at any rate when induced by illuminating gas. 

On the other hand, a more extensive examination of abscission in 
certain plants indicates that all evidences of increased turgor may at 


380 Unwersity of Califorma Publications in Botany [ Vou. 5 


times be absent. Such cases might be explained by the absence of 
any considerable amount of starch in the cells concerned. Indeed, 
the starch grains usually noted in the separation layer can not at 
times be observed. This might also explain the fact that the bulging 
of the epidermis and collapse of cells in the pith usually acecompany- 
ing abscission are sometimes absent. Also, starch grains are rarely 
observed in the separation cells of Lycopersicum and Datura and in 
these forms very little bulging of the epidermis occurs. Although 
humid conditions favor abscission and drought prevents the process, 
it has also been observed that drought has to be very severe before it 
produces such a result. Other evidences for increased turgor derived 
from the turgid appearance of the cells are mostly obtained after 
abscission has started and, granting that the cells are isolated by dis- 
solution of the middle lamella, more or less expansion due to release 
of pressure is to be expected. 

A critical examination of the separation cells during abscission 
brings out several facts, other than those mentioned in the above 
paragraph, which of themselves render inadmissible the theory that 
cell separation is brought about by increased turgor. These are 
as follows: 1. There seems to be no perceptible change in cell 
shape or size during separation. 2. The increase in size of the inter- 
cellular spaces does not necessarily take place first between the walls 
at the ‘‘corners’’ 
streak between the lateral walls of the cells (pl. 51). 3. Cell isola- 
tion may be incomplete in large numbers of cells still remaining 
attached to each other. 4. Cell separation first becomes complete in 
a narrow plane between only two tiers of cells before spreading later 
to a larger number of cells. 5. The spreading of cell separation 
itself is obviously hard to explain on the basis of the turgor theory. 


of the cells, but may appear first as a longitudinal 


In view of the facts brought out in this discussion and the positive 
evidence for the dissolution of the primary membranes, it should be 
clear that increase in turgor, at least in the Solanaceae, is not the 
direct cause of cell separation. Undoubtedly there is often great 
increase in turgor during abscission, especially in certain types of 
cells, but this increase, instead of being the direct initiating factor, 
probably serves merely to hasten and facilitate the process. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 381 


e. EXPERIMENTS ON THE AMOUNT OF SUGAR IN THE STEM 
AND PEDICEL OF NICOTIANA DURING ABSCISSION 


After several experiments, all of which indicated the results ob- 
tained below, the following experiment was performed. Experiment 
2a was devised to show the change in the amount of sugar which 
occurs in the tissues of the pedicel during abscission. Experiment 2b 
was intended to show this same difference in a restricted region of the 
stem just proximal to the separation layer. 

Experiment 2a. Lot A included 200 pedicels of flowers which had 
fallen a few minutes before being collected as a result of being sub- 
jected to illuminating gas. Lot B ineluded 200 pedicels of flowers 
picked at the same time as those making up Lot A, but in which no. 
abscission was induced. The water extracts made with 10 ec. from 
equal weights of the two lots were tested with surplus Fehlings solu- 
tion. The precipitates formed upon boiling weighed: 


Experiment 2b. This experiment was carried out in the same man- 
ner as experiment 2a, but the precipitates in this case were of such 
small quantity that no attempt was made to get actual figures as to 
their weights. It was clear, however, merely from an examination of 
the filter paper, that there was more precipitate in B than in A—just 
the reverse of Experiment 2a. The difference was evidently not as 
great as in the latter experiment. 

Experiment 2a seems to indicate that during abscission there is a 
reduction of nearly one-third the normal amount of sugar in the 
pedicel. Other preliminary experiments performed as abscission was 
starting showed only a slight reduction in the amount of sugar in the 
pedicel. Thus possibly the withdrawal of sugar commences with the 
start of abscission. Experiment 2b indicates that there is probably a 
slight increase in the amount of sugar in the limited region proximal 
to the separation during abscission. It is possible that most of the 
withdrawn sugar is used as a source for the energy required in the 
active process of cell separation. The slight increase proximal to the 
separation layer also shows that there is probably an increase in cell 
turgor in the actual tissues which contain the separation layer, due 
to the conversion of starch into sugar. 


382 Unwersity of California Publications in Botany [Vo. 5 


f. POSSIBLE AGENCY ACTIVE IN THE DISSOLUTION OF 
THE MIDDLE LAMELLA 


The pectose of the middle lamella may be broken down into the 
soluble pectin in three different ways—by the action of an acid, 
of an alkah, or of the enzyme pectosinase. Since it is doubtful 
whether alkaline reactions in living cells frequently get strong enough 
to affect the middle lamella, the probable active agency is limited 
to the acid or the enzyme action. Up to the last few years very 
little has been known about the action of enzymes concerned in 
pectic digestion. It has been natural, therefore, for investigators 
(ef. Wiesner, 1905, and Kubart, 1906) to consider the acid as prob- 
ably the active agency. In this connection, it is well to state that I 
have obtained distinct acid reactions with litmus from the base of the 
corolla of Nicotiana during abscission. This would confirm Kubart, 
who, it will be remembered, obtained similar reactions from the corolla 
of Nicotiana. But in this ease I sometimes obtained acid reactions 
from the corolla when in the normal condition. Since these observa- 
tions offer no detailed evidence that acidity has increased during 
abscission to a degree higher than normal, their significance can well 
be doubted. 

The tissues of Datura give a distinct acid reaction to litmus in the 
normal condition. Experiment 3 below shows a slight inerease in 
acidity during abscission. No acid reactions of much intensity are 
given by the base of the pedicel of Nicotiana either in the normal or 
abscissed condition. 

Experiment 3. Lot A contained the bases of three pedicels cut 
while abscission was going on. Lot B contained an equal weight 
(6 em.) of the bases of three pedicels cut in the normal condition. 
These were extracted with water and the extracts made up to 10 ce. 
each. By titration with 10 per cent NaOH and phenolphtalein the 
following results were obtained : 


IAS. seat Se ck To is 0.75 ee. required to neutralize 
Ee Se eer 0.6 ee. required to neutralize 


A similar experiment on Nicotiana showed, however, that the nor- 
mally low acidity of this genus is shghtly reduced during abscission, 
as indicated by the following results: 


BN ase eee eee 0.25 ec. required to neutralize 
UES ise cee eee eae 0.87 ee. required to neutralize 


1918| Kendall: Abscission of Flowers and Fruits in Solanaceae 383 

The normal acidity in Datura is high, but it is doubtful whether 
the increase is large enough to account for the dissolution of the 
At any rate, it is certain that acidity does not enter 


middle lamella. 
We must, therefore, 


into the problem in the pedicel of Nicotiana. 
fall back upon the enzyme action as probably responsible for the 
process of cell separation. 

Most hydrolysing processes characteristic of living cells are now 
supposed to be due to the action of enzymes of different kinds. It 
has been definitely claimed (ef. Atkins, 1916) that an enzyme which 
has been called pectosinase is capable of breaking down the pectose 
of which the middle lamella is composed. Add to this the fact that the 
action of enzymes has been shown, as has also the process of abscission, 
to be very sensitive to all kinds of changes in the external environment, 
and it is fairly safe to assume that the method of cell separation is 
fundamentally an enzyme problem. Irrefutable proof of this could 
be obtained only by testing for the activity of pectosinase during the 
early stages of abscission and by demonstrating the absence or in- 
activity of this enzyme in species where abscission does not occur. 


ABSCISSION OF THE STYLE AND COROLLA 


Abscission of the corolla in Nicotiana was described by Kubart 
and it may be said at once that the observations herein described 
agree entirely with his. Abscission of the 
corolla is brought about by the separation, 
without any previous cell divisions or 
elongations, of living cells at the base of 
the corolla tube. The separation layer, 
which is in no way morphologically differ- 
entiated from the neighboring tissue, 1s 
located about 1 mm. from the point of in- 
sertion of the corolla on the receptacle. It 
thus oceurs in the distal part of a region 


(f 


Ce) 
U7, 
a 


LT 


CTT 


rnin 


where intergradation of cell shape, between 
the isodiametric cells of the receptacle 
and the more or less elongated cells of the 


corolla, is apparent. The separation cells 
which are in this region of intergradation 
are not isodiametric but are more or less 
elongated parallel to the long axis of the 
corolla. All the cells in cross-section of the 


Fig. 6. Longitudinal radial 
section of the base of the 
corolla tube of Nicotiana, 
showing the method of ab- 
scission. 


384 Unwersity of Califorma Publications in Botany [ Von. 5 


base of the corolla tube at about the level of the separation layer seem 
to be involved in the process except the epidermal cells and the 
tracheew. The process of cell isolation in this case may spread up and 
down for quite a distance between the epidermis and trachex, thus 
involving a large number of cells (fig. 6). 

Abscission of the corolla in Datura differs slightly from that in 
Nicotiana. As in the latter, there is no differentiated separation 
layer, separation occurring in cells which are not visibly different 
from other cells of the corolla. Cells more or less elongated are in- 
volved, as in Nicotiana, but in Datwra the region of separating cells 
is limited to certain tissues—that is to say, not all the cells across 
the base of the corolla tube at about the level of the separation layer 


are involved in the process of abscission. The base of the corolla in 
Datura is characterized by distinct longitudinal ridges which alternate 
with deep grooves. Thus, a cross-section of a portion of the base of 
the corolla appears as in fig. 8. Cell separation fails to occur in the 
outside ridges at the level of the separation layer, so that, looking at 
the base of the corolla tube from the outside during abscission, one 
sees separate crescent-shaped regions of macerating cells alternating 
with cells which are not separating (fig. 7). This is explamed when 
a cross-section is taken (fig. 8), which shows that several vascular 
bundles, the célls of which do not sep- 
arate, are collected in the outside ridges. 
Abscission of the style occurs nor- 
mally in Nicotiana and Datura a short 
time before the corolla has fallen. So 
far as it was possible to determine, the 
process of abscission is exactly the same 
in the style as in the corolla. A separa- | 
tion of very small, more or less elongated 
cells takes place at the base of the style 


without any external indication such as Fig. 7 
frequently occurs in the pedicel of the c—region of separating cells. 


Gane 
Fig. 8 

a—vascular bundle. 

c—region of cell separation. 
b—region of no cell separation. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 385 


flower. As in the case of the corolla, there is no structure which might 
be interpreted as a differentiated separation layer. Separation occurs 
im a region of intergradation in cell shape between the spherical cells 
of the ovary and the eells of the style, which are elongated parallel 
with the long axis of the style. 


TIME OF ABSCISSION 


1. Reaction TIME 


a. REACTION TIME IN NORMAL ABSCISSION 


ce ? 


The term ‘‘reaction time’’ is used in referring to two rather dis- 
tinct subjects. First, we may have a reaction time represented by 
the time intervening between anthesis and normal abscission due to 
lack of fertilization. Second, we may have a reaction time which has 
to do with the period between the application of the stimulus and 
flower-fall in ‘‘spontaneous’’ abscission. The reaction times in normal 
abscission were discussed in an earlier communication in the case of 
two F, species hybrids of Nicotiana and their parents. The state- 
ments there made have been repeatedly verified and in addition a 
considerable amount of data has been accumulated in regard to the 
time of abscission in other species of Nicotiana and in the genus 
Lycopersicum. In the ease of the former the observations were also 
made upon the abscission of the corolla, the effect of pollination on 


oe 


reaction time, and the reaction time in ‘‘spontaneous”’ abscission. 

In determining the abscission times for the hybrid F, H154 (ef. 
page 386), a great variation was noted in the normal reaction time. In 
the case of the hybrid F, H179 (ef. page 386), however, and in other 
species or varieties investigated, very little variation in the time of 
abscission has been noted. The range of variation in these species 
and varieties practically always falls within two or three days and a 
large number of observations gives identical times, as far as the number 
of days is concerned, in the ease of seven to ten flowers. There is a cer- 
tain variation in the length of the reaction times in different flowers on 
the same plant, but the plants of a species do not differ from one an- 
other in their average reaction times. It was noticed that the figures 
were approximately the same whether the averages were based on the 
records of four or five flowers or of a considerably larger number; 
thus the results given in the following table may be considered con- 
elusive. Where the number of flowers involved is less than four, 


386 University of California Publications in Botany _— [Vous 


however, the results serve merely as approximate estimate of the 
abscission reaction time. 

In obtaining the records tabulated below a separate tag was sup- 
pled for each flower. This tag was put on the flower at the begin- 
ning of the observation, the plant visited twice a day thereafter and 
records kept on the tags, which were left on the flower until the close 
of the observation. If a flower fell upon being tapped or shaken, 
abscission was considered to have occurred and the date was recorded 
on the tag, which was then collected. Similarly, in the case of the 
records for abscission of the corolla, a slight pull had to be applied 
before it could be determined whether abscission had occurred. As a 
means of preventing fertilization, the stigma was cut away in addi- 


TABLE 1 
II Ill IV Vv: 
Time from Time from Time from Time from Time from 
bud! to pollination to | pollination to anthesis to anthesis to 
anthesis mature fruit abscission of abscission of | normal flower*® 
Designation of corolla corolla, fall 
species or variety unpollinated 
Toe Avg. No. Avg. c Avg. No. Avg. No. Avg. 
fovere| ee flowers ae dees anes flowers ae flowers Nee 
F, H38 10 18 
5 9 9 7 
F, H179 4 8 4 14 15 3} 2 : 20 
F, H36 7 4 3 6 
a 10 11 6 4 9 6 9 15 
N. sylvestris 4 6 
N. Tabacum 3 12 2 4 2 6 3 
““Maryland’’ 
N. Bigelovii 5 10 5 3 5 5 8 1 
var. Wallacei 
N. Bigelovii 5 19 5 2 6 5 6 jno fall 
““Pomo’’ 
N. Bigelovii { 2 16 
var. typica 3 |no fall 
N. Bigelovii 4 21 1 4 2 8 
(hybrid?) 
N. multivalvis 3 18 2 4 2 |no fall 
N. quadrivalvis 5 20 5 7 5 2 6 jno fall 
N. suaveolens 8 8 8 8 1 13 
N. Sanderae 15 15 5 4 5 6 6 9 
N. rustica var. 3 4 + 6 
brasilia 
N. rustica Vine 0 6 7 Silene: 4 
(Winnebago) | 
N. rustica var.? 4 15 4 2 5 3 6 5 
Lycopersicum 
esculentum 4 6 4 3 4 8 Wye 9 


1The buds recorded here were of such size that the corolla and calyx were of the 


same length. 
?Sterile pollen applied to the stigma. 
* By normal flower-fall is meant fall due to lack of fertilization. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 387 


tion to removing the anthers before anthesis. Various experiments 
had shown that such an operation on the flower does not induce 
abscission or affect its normal physiological condition to any great 
extent. 

F, H154 is Nicotiana Tabacum var. macrophylla (U. C. B. G. 
22/07) & N. sylvestris (U. C. B. G. 69/07). F, H179 is N. Tabacum 
**Cuba’’ (U. C. B. G. 200/14) & N. sylvestris. F, H36 is N. sylvestris 
«x N. Tabacum var. angustifolia (U. C. G. B. 68/07). 

The results given in table 1 indicate, in the first place, that the 
different species differ considerably in all the types of abscission 
reaction times considered, and, in the second place, that on the aver- 
age, application of a fertile pollen to the stigma tends to shorten 
the time between anthesis and abscission of the corolla by two days. 
The one apparent exception to this statement is Nicotiana suaveolens, 
but in this case the pollinated flowers fell five or six days later 
than the corolla, indicating that growth of the pollen had not pro- 
ceeded very far. Records on F, H179 and N. sylvestris indicate that 
sterile pollen does not have the same effect on abscission that fertile 
pollen does. This would seem to show that here the effect of pollina- 
tion upon the postfloration phenomena is not due, as Fitting (1909) 
has found in orchids, to mere mechanical or chemical stimulation of 
the stigma by the pollen. This much being certain, the question still 
remains whether the results obtained depend upon fertilization or are 
due to the growth of the pollen tubes down through the style. 

According to East (1915), working on self-sterility in Nicotiana 
hybrids, the pollen tubes reach the ovary, in cases of cross-pollination, 
three or four days after application of pollen to the stigma. Since in 
all cases recorded above cross-pollination was carried on and since in 
most cases the corolla was not thrown off until three or four days 
after application of pollen to the stigma, it is possible that fertiliza- 
tion is the important factor in shortening the time between anthesis 
and abscission of the corolla. In N. quadrivalvis, however, the corolla 
was thrown off within eighteen hours after pollination, whereas, when 
pollination is prevented, the corolla may remain on the flower for 
fifty-seven hours. If East’s conclusions are correct, this would seem 
to indicate that the shortening of the reaction time in abscission of 
the corolla is due to some stimulation of the style by the pollen tubes 
and not to fertilization. This conclusion, however, could be doubted 
even here, because the style of N. quadrivalvis is very short, so that 
the pollen tubes might reach the ovary in a much shorter time than is 


388 University of California Publications in Botany [Von. 5 


required in larger flowers. An attempt was made to get further data 
on this point by removing the style several hours after application of 
pollen before the pollen tubes could possibly have reached the ovary. 
This operation occasionally causes the whole flower to fall, and since 
in such eases abscission in the pedicel occurs before fall of the corolla, 
no results in regard to the latter organ are obtained. The possible 
effect of the operation on the abscission of the corolla was checked 
by control tests of unpollinated flowers in which the styles had also 
been removed. This can also be checked by a comparison with the 
periods of time given in table 1, column III. 

It was found in three flowers of F, H179 that, when the style was 
removed three days after pollination, the corolla was, on the average, 
thrown off three days after anthesis. The control test for this experi- 
ment gave in three flowers an average of five days. Where the style 
was removed two days after anthesis, four flowers gave an average of 
three days. Where the style was removed one day after pollination, 
the corolla was abscissed in five flowers an average of three days after 
anthesis. A control test gave in this case an average of five days for 
five flowers. Finally, the style was removed in seven flowers seventeen 
hours after pollination. The seven flowers gave in this ease an average 
of four days for the time between anthesis and fall of the corolla. A 
control test for this last ease gave for five flowers an average of five 
days. 

These experiments were repeated with N. sylvestris. In one ease 
where the style was removed in three flowers two days after pollina- 
tion, the corolla was thrown off on an average of four days after 
anthesis. A control test of this case gave an average of six days for 
three flowers. In another case the style was removed in three flowers 
one day after pollination. In this case the corolla was abscissed on 
an average of three days after anthesis. 

The results given in the above paragraphs indicate definitely that 
it is the stimulation of stylar tissues caused by the growth of the 
pollen tubes which shortens the time between anthesis and abscission 
of the corolla. They also show that the removal of the style has no 
appreciable effect on the abscission of the corolla. It is evident from 
the results given that the influence of the pollen is seen as early as 
seventeen hours after pollination, and it is possible that the effect may 
be manifested even earlier. It is significant that the period given in 
the ease where the style was removed seventeen hours after pollina- 
tion is one day longer than in the ease where the style was removed 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 389 


twenty-four hours after pollination. This may possibly indicate that 
in the first ease the influence of the pollen tubes has diminished, be- 
cause of the shortening of the period which they have had for growth. 
If this is the case, it is reasonable to suppose that the influence of the 
growing pollen tube increases up to twenty-four hours after pollina- 
tion as the pollen tube lengthens. Thus, at six hours after pollination 
it is possible that no effect of the pollen tubes would be noticeable, 
while twenty-four hours after pollination the entire influence of the 
growing pollen tube has been exerted. 

The effect of pollination on the time between anthesis and flower- 
fall was tested by experiments similar to those described above. 
Results in such experiments are difficult to obtain because removal of 
the style frequently causes the premature fall of the flower. If the 
flower fell before abscission of the corolla, the fall was considered 
premature, as the result of the removal of the style, and the record of 
that particular flower not considered. Since under ordinary condi- 
tions pollinated flowers remain on the plant, it is to be expected that 
the stimulation of the stylar tissues by the pollen tubes, if it has any 
influence at all, would increase the length of time between anthesis 
and flower-fall. Granting the truth of this assumption, any reduction 
in time between anthesis and fall can be considered as the result of 
removal of the style. 

In one test on ten flowers of F, H179, where the style was removed 
two days after pollination, flower-fall occurred on an average of seven 
days after anthesis. A control test in this case also gave seven days 
for ten flowers. This time is approximately the same (the actual 
average calculated to the tenth of a day was 6.7) as those given in 
table 1, column V, for the time between anthesis and normal flower- 
fall due to lack of fertilization. A similar test on six flowers of N. syl- 
vestris, where the style was remoyed two days after pollination, gave 
an average of thirteen days. The time for this species in table 1, 
column V, is fifteen days. 

These two records indicate that the stimulation of the stylar tissues 
by the growing pollen tubes has no effect on the time between anthesis 
and flower-fall. In the second case above, and also perhaps in the 
first, the stimulation of the style seems to have shortened the time 
somewhat, but in this case the result can be explained by the effect 
of the later removal of the style. 


390 University of Californa Publications in Botany [ Vou. 5 


b. REACTION TIME IN ‘‘SPONTANEOUS’’ ABSCISSION 


Exact data in regard to the reaction time can be given only in 
two definite cases. The observations in these cases were made on 
small shoots of the plant to be considered, which were placed in water 
and inserted under a bell-jar containing 1.5 per cent illuminating 
gas. After several hours, the material was shaken every fifteen min- 
utes to determine when the first flower fell. F, H179 and N. Tabacwmn 


2? 


““Maryland’’ were selected as material for the experiments because 
these forms were found most sensitive and thus react regularly and 
quickly to stimul. <Abscission occurs in the pedicel of F, H179 seven 
hours after insertion into 1.5 per cent illuminating gas at a tempera- 
ture of approximately 19° C. The smaller buds begin to fall first, 
but are followed in a short time by the open flowers. Abscission 


in eight hours under the above 


bi) 


oceurs in NV. Tabacum ‘‘ Marylanc 
conditions. 

The remainder of the data having to do with the reaction time in 
spontaneous abscission is in the form of approximate estimates derived 
from the results of experiments on the induction of abscission. In 
the case of abscission induced by illuminating gas most species which 
shed their flowers in 1.5 per cent illuminating gas do so after ten or 
fifteen hours at room temperature. 

There remains now to be considered the reaction time in cases of 
flower-fall due to mechanical injury. The results along this line are 
largely derived from tables 2, 3, 4, and 5, which, however, were 
arranged to show more particularly the comparative effect of different 
types of injury, as causing or not causing abscission in flowers of 
various ages. These tables might as well be presented under the 
heading ‘‘Experimental Induction of Abscission by Mechanical In- 
jury’’ (page 405), but since it is necessary to draw certain conclusions 
from them in regard to the time of abscission they are presented and 
explained at this time. 

Tables 2, 3, 4, and 5, which follow, serve to record the results of 
a number and variety of experiments all designed to show the relation 
of mechanical injury to abscission. It was very soon discovered while 
carrying on the experiments that the effect of injury depends to a 
large extent upon the age of the flower. Now the age of the flower 
can be most conveniently measured by determining the increase in 
size of growing parts such as the corolla and ovary. Thus it was 
necessary in each case to record the size of the flower—size being a 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 391 


eriterion of age—upon which the test was being made. This was done 
by noting on the tag which was supplied for each flower (ef. page 385) 
the length of the corolla in millimeters, the condition of the corolla, 
or any other condition of the flower which would serve to indicate its 
age. The period of development of the flower and fruit is divided 
into several arbitrary stages, each of which is designated by a Roman 
numeral in the second column of the tables. Where the number of 
flowers designated in the first column are nearly in the same stage 
of development only one numeral appears in the table, but where the 
range in size of the flowers is quite extensive two numerals appear, 
representing the range in size within which the flowers were found at 
the time of the experiment. The stages of floral development which 
each Roman numeral represents are given below. 


Bud 
| ee eee corolla 2mm. to 5 mm. in length 
1 eae eee ee corolla 6 mm. to 10 mm. in length 
10 0 eee eee corolla 11 mm. to 15 mm. in length 
NB Vier Pec nea cso corolla 16 mm. to 20 mm. in length 
Witte Fosse tee ee corolla 21 mm. to 30 mm. in length 
AV Te tot Ses aa corolla 31 mm. to 40 mm. in length 
(Vee eee ee corolla 41 mm. to 50 mm, in length 
Flower 
NY ee eeeeeresene em corolla opening : 
1D eee eee anthesis 
PXCW es as 2 days after anthesis 
PNT Fe) cess Aas corolla withering 
Fruit 
Immature 
2G Dieter eee fruit 5 mm. to 8 mm. in length 
D211 I) Eoin fruit 9 mm. to 10 mm. in length 
Mature 
ORV iach eee Sot fruit 11 mm. to 12 mm. in length 


The operation of injuring the flower consisted largely in removing, 
by cutting away with a sharp safety razor blade, entire floral organs 
or parts of them. In some eases, however, organs were only slit longi- 
tudinally with a sharp knife or merely punctured with the point of a 
pair of forceps. 

Several types of injury that remove the style, stigma or stamens 
before pollination may cause fall by preventing fertilization. It is 
evident, therefore, that fall occurring after such an operation per- 
formed on the flower before anthesis may be due to lack of fertiliza- 
tion and not to the injury. If, however, the fall occurs within the 
minimum time elapsing between anthesis and normal flower-fall due 


392 University of California Publications in Botany 


[ Vou. 5 


to lack of fertilization, it can be safely concluded that the fall is due 


to the effect of the injury. 
for N. Langsdorffi. 


This minimum time is about seven days 
It can be safely said, therefore, that any fall 


occurring in less than seven days after injury to the flower near 


anthesis is due directly to the effect of the injury. 


In eases where the 


stamens or style are removed in flowers younger than those at anthesis, 


TABLE 2 
EFFECT OF DIFFERENT TYPES OF INJURY IN CAUSING FLOWER FALL IN 


N. Langsdorffii var. grandiflora 


Avg. No. 
No. Size or Injury to cava eure 
flowers } conditions) : = benianine 
Calyx Corolla Stamens | Pistil Pedicel | organs 
| 
10 | I-VI | alleut | all eut | all cut all cut } il 
a 10 VII-XI | oe oe oe oe } wy 
10 | XII-XIIT a oe zs uy 
2 XIV | BG | a ae oe no fall 
4 I-IT + cut # eut te | style cut 7 
3 III-VII te oe oe oe 7 
3 Win bx | ce iG “cc “cc 7, 
b4 5 | XI-XII | ee Ob fe | As no fall 
} 20 XX | ae a ou | style and 5 
part of 
| ovary cut 
3 |XIII-XIV GG uO BM ee 4 
r § | V-VII + eut es ,# style cut 7 
) By NANI De) BG of ef | 6 
e4 9 XI | oe oe oe 9 
3 XII OG as | a | no fall 
( 4 I | all eut 3 
Gly al Il | Gt no fall 
(12) nese ub “ 
(2) wear sliton sides) 2 slit on 7 
| to base 2 sides 
| to base 
WS |b WGieS-ap || OY GG | no fall 
ens 104 z cs ovary slit 3 
3 V-VII GG Ke a 5 
3 IX “cc m3 ee | 9 
We XIT | ne ie yy 5 
fap IDG anthers | 10 
t. all cut 
} 2: V-VII all eut ii 
{ 3 VII-X ee | 9 
4 Vill stigma cut 9 
[ay V-VII style cut Tf 
2 VII-X Me 10 
ved XIV all eut all eut no fall 
ni 2 XIII 4 cut + eut 2 
) 2 XIV es ee no fall 
4 |XIII-XIV| slit to base slit to base UO 
(10 II slit to base ue 
3 IX se a 
qi} @)) Mewanm 1 eut iy 
| + through 
8 | II-XII 2 euts a 
| | + through 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 393 


allowance must be made for the approximate number of days preced- 
ing anthesis. Thus, if a flower of the above species is injured three 
days before anthesis, the fall can not be assigned to the injury unless 
it occurs before ten days have elapsed. The minimum time for 
F, H179 is about five days; thus, any time of five days or more recorded 
on a flower, injured near anthesis, was considered as ‘‘no fall.’’ The 
minimum time for Lycopersicum is about six days. 

Finally, it is necessary to state that the process of reaction to the 
different types of injury recorded in the following tables was by no 
means impeded by low temperatures. Nicotiana Langsdorffii was 
tested out in a greenhouse where the average temperature approxi- 
mated 75° F. The tests on F, H179 and Lycopersicum were per- 
formed in the botanical garden of the University during July and 
August, when the temperature was also comparatively high. 

The following statement of results is derived in great part but not 
entirely from the foregoing tables. It has been noticed that cutting 
off the freshly opened flower at the tip of the pedicel causes the 
remainder of the pedicel to be thrown off m from ten to fifteen hours, 
but after the same operation on developed capsules the pedicel re- 
mains firm from thirty-six to ninety-six hours after the injury. 
Removal of the calyx causes the fall of buds in two or three days, 
depending upon the age of the bud. Removal of half the calyx 
together with two-thirds of the corolla and all the stamens causes 
fall in one to four days, depending upon the age of the flower. A 


TABLE 3 
EFFECT OF POLLINATION OF FLOWERS or N. Langsdorffii var. grandiflora on 
REACTION TO INJURY 


No | Avg. No. 


flowers Pollination | Injury days Betore 
a 
af 2 pollinated when injured calyx and stamens eut no fall 
oiled not pollinated ee 10 
in) & pollinated when injured ealyx ‘* 4 corolla cut no fall 
y5: not pollinated fe 8 
No. days after pollination when 
injured } 
{ 2 il | all organs cut at tip of pedicel 2 
| 2 2-6 | HY 2 
e412 TS GG 2 
| 3 2 34 calyx, #% corolla, stamens, 4 
style eut 
3) 45 5 
Ih D) 6=7 ce 5 
l is) 9 G6 no fall 


394 [ Vou. 5 


Unwersity of Califorma Publications in Botany 
transverse cut through the entire flower which passes through the 
middle of the ovary causes fall in one to two days. A similar oper- 
ation in the ease of maturing fruits changes the date of fall to 
four to eight days. Removal of half the corolla and all the stamens 
causes fall of buds in one day and the fall of young flowers in two to 
three days. Removal of the stamens or style in buds causes fall in 


TABLE 4 
EFFECT OF DIFFERENT TYPES OF INJURY IN CAUSING FLOWER FALL IN F,H179 
4 Avg. No. 
Nowa ecaon py eee 
flowers | of flowers remaining 
Calyx Corolla Stamens Pistil | Pedicel organs 
{ 9 | 1I-VUI 4 cut $ eut all cut | style eut 1 
EU @ || SOY as a a ee no fall 
b f 4 | IIl-VII + cut SS 1 
(10 | VIII-IX a ms | no fall 
e 10 | V-VIII oe | Ws 
2 I all eut | 2 
7 II as | 9 
| 3 II - | no fall 
ad 4 | Ill-Iv oe | 3 
6 | III-IV . | no fall 
1 Vv He | 2 
4 | V-VII os no fall 
L 2 IX 6c | “cc 
[ 7 | Ill-Iv os 2 
1 V SY 5 
oye 3 Vv is no fall 
l 6 | vi-vi as | sg 
t{ 5 | II-VIII ey 2 
4 | VII-VIII ae | no fall 
if al II 1 slit on 2 | 1 slit on 2 } 5 
sides to sides to | 
base base | 
1 II - a | no fall 
| 9 IV-VII ce “ec | oe 
81 9 II 2 slits on 2/2 slits on 2 1 
sides to sides to 
base base | 
2 II-IV a M 4 
| 5 | v-vIr “ “ | no fall 
{( 5 II-V punctured | punctured ovary 2 
| on both on both punctured, 
sides sides small hole 
hi 3 | VI-VII “ g oH no fall 
| 3 | VII-XI ee 5 ve 2 
3 T1-Il oi a “iN 2 
l 3 VI-X “ec “cc “a 2 
15 | IlI-XII 1 slit to | no fall 
base 
i; 5 /punctur’d 1 
| | many 
l times 
6 XIV 4 eut | capsule 4 
j { 4 cut 
3 XIV sr re no fall 
| 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 395 


two to four days. Severe injury of any kind to the ovary causes fall 
in one to two days. 

The figures given above for the reaction time in eases of abscission 
following mechanical injury, together with a more detailed considera- 
tion of the tables, indicate that the reaction time, in general, does not 
depend so much on the type of injury as on the age of the flower 
concerned. What connection there is between the type of injury and 
the reaction time seems to be based, except in cases of injury to the 
ovary, on the relation of the amount of material removed to the 
amount remaining. Thus, cutting off the flower at the tip of the 
pedicel causes abscission of the remaining pedicel more quickly than 
any other type of injury. One exception to this statement is seen, as 


TABLE 5 


EFrect OF DIFFERENT TYPES OF INJURY IN CAUSING FLOWER FALL IN 


Lycopersicum esculentum 


Avg. No. 
No. pieicr Injury to days veto 
flowers petiowers | Fon ee ] ry 7 a cae ae 
Calyx | Corolla Stamens Pistil Pedicel organs 
{ + I alleut | no fall 
a; 4 | II-VII .: a 
l 6 XIL ve ac 
(3s XII | entire 2 
ovary cut | 
3 |XIII-XIV, | ovary no fall 
: punctured 
| | 4 times on 
b | | top 
1 | Se 3 
| 4 XII | ovary 2 
punctured 
4 times on 
side 
L 3 XIV a no fall 
{ 4 Il punctured | punctured ovary &) 
| at base punctured 
e | once on 
| side 
l 4 VIII se oe ue 4 
( 4 | II-VIII 4 cut 4 cut no fall 
| 4: | VIII-IXx | in st i 5 
d{ 3 I-Il | N ovary 4 eut 1 
l 3 VIII ee ee oe 3 
9 ID } vc “cc (a3 9 
e 5 I-IX | uP all eut no fall 
r{ 5 VIIt style cut 5 
6 X-XI se no fall 
g 5 |VII-XIV slit uC 
3 VIll all cut is 4 
hy 5 oe of no fall 
4 | II-VIII 2 


396 University of California Publications in Botany [Vou. 5 


indicated above, in the case of injury to the ovary in which this organ 
may be merely punctured, without necessarily removing any material, 
yet abscission occurs in one to two days after the injury. 

It has, on the other hand, been evident throughout all the abscis- 
sion experiments that age of flower is the important factor in deter- 
mining the reaction time, older flowers nearly always responding more 
slowly to stimulation by injury than younger ones. It will be seen, 
however, from the tables that there are occasionally individual excep- 
tions to the general rule. These exceptions might be explained in a 
number of ways. For example, it is possible in the case of older flow- 
ers that the ovary, having increased in size, was accidentally cut in 
the operation of injury, thus adding the extra factor of stimulation 
of the ovary which in younger flowers would not be present. In gen- 
eral, such exceptions to the general rule indicate to what extent the 
normal or abnormal physiological conditions of the plant enter into 
the problem. 


2. ABSCISSION TIME 


The abscission time, or the actual time involved in the process of 
cell separation, was considered in a preliminary paper (Goodspeed and 
Kendall, 1916) wherein the minimum time in which abscission was 
known to have occurred was stated to be from four to eight hours in 
normal abscission and from one to four hours in ‘“‘spontaneous’’ 
abscission. A few additional data are now at hand in the ease of 
F, H179 and Nicotiana Tabacum ‘‘Maryland.’’ These two forms, 
as has already been noted, are a little more sensitive than most 
Nicotiana varieties and normal abscission was found to take place in 
from three to six hours. 

The time of cell separation in 
more exactly determined than that in normal abscission because of 
the regularity with which the plants respond to certain conditions of 
injury or to the presence of narcotic vapors. Data on this point were 
obtained in the following manner. Flowering shoots with flowers of 
different sizes were cut, placed in water and inserted under a bell-jar. 
Enough illuminating gas was then introduced under the jar to make 
1.5 per cent approximately. The temperature during the experiment 
was practically constant at 19° C. After the shoot had been left in 


f 


‘spontaneous’’ abscission can be 


this abnormal atmosphere for five hours a few flowers were picked off 
at fifteen-minute intervals and free-hand sections made of their 
pedicels until flowers about the size of those which were being sec- 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 3897 


tioned began to fall. It was found that signs of abscission hardly 
ever appeared until thirty to forty minutes before actual fall occurred. 
This indicates that the actual process of cell separation in F, H179 
takes place in from thirty to forty minutes. Experiments carried on 
in the same manner with N. Tabacum ‘‘Maryland’’ indicate that 
abscission here takes place in from forty-five to sixty minutes. 

Both the reaction time of abscission and the actual abscission time 
are profoundly influenced by temperature and by humidity. Varia- 
tion in the intensity of the illumination, however, seems to have no 
direct influence upon abscission. In comparing the effect of changes 
in temperature and humidity it was found that the results of experi- 
ments intended to show the time of abscission are far more dependent 
upon temperature than upon humidity. This is not because changes 
in humidity have little influence upon abscission but because such 
changes have to be very great indeed before bringing about any appre- 
ciable effect. Very slight changes in temperature, on the other hand, 
often influence abscission to a marked degree. Abscission goes on 
very actively under high temperatures and conversely very slowly 
under low temperatures. It starts in the case of F, H179 about seven 
hours after insertion in 1.5 per cent illuminating gas at a temperature 
of 19° C. If the same experiment be repeated in a temperature of 
approximately 9° C. abscission may not occur for fifteen to twenty- 
four hours. 

Drought has to be quite severe before retarding abscission. There 
is no doubt, however, that wilted shoots will not drop flowers as 
quickly as fresh ones and if the wilting proceeds far enough no abscis- 
sion will oceur. This effect is all the more noticeable if the air around 
the wilted shoot is kept free from moisture. 


EXPERIMENTAL INDUCTION OF ABSCISSION 
1. Inpuction By ILLUMINATING GAS 


The first subject to be considered under this heading is the com- 
parative effect of illuminating gas in causing abscission in several 
species of the Solanaceae. The method of determining this consisted 
largely in placing flowering shoots of the different species in water 
under bell-jars and introducing enough illuminating gas under the 
jars to make the percentage of narcotic vapors in the air around the 
plant 1.5. The temperature during the experiments was compara- 


398 Unwersity of California Publications in Botany [Vou. 5 


tively high, ranging from 15° to 20° C. The results, which were 
recorded approximately fifteen hours after subjection to the gas, are 
given in the following table: 


TABLE 6 
Species, variety, or Amount of abscission, expressed almost entirely 
hybrid in terms of size of flowers thrown off 


N. Tabacum var. macrophylla.....all buds up to anthesis. 
N. Tabacum ‘‘ Maryland’? .......all flowers up to 4 or 5 days past anthesis. 


F, H154 ... all buds up to opening of corolla. 
F, H36 . .all buds and flowers. 
F, H179 _. all buds and flowers. 


N. glauca 
N. rustica var. 
N. rustica var.? 


young buds. 
- buds up to anthesis. 
buds, flowers, and fruits. 


N. Bigelovii var. Wallacei.......... no abscission. 
N. Bigelovii ‘‘Pomo’’ . abscission. 
ING (quiardinivalivis Sess ee een eee abscission. 
ING tise iwallS eens nee eet abscission. 


N. Sanderae - buds up to anthesis. 

N. suaveolens buds up to anthesis. 

N. plumbaginifolia —.....--. buds up to opening of the corolla. 
Solanum umbelliferum -............ »small buds. 

S. jasminioides buds and flowers. 

S. verbascifolium ...................... no abscission. 

---- small buds. 

---- no abscission. 


S. nigrum 
Iochroma tuberosa 


Cestrum fasciculatum —............ . buds and flowers. 
Lycopersicum esculentum var. 
sy AGI OTN Crees ccc eeeseeeeneeeee no abscission. 
L. esculentum var. vulgare ..... small buds and occasional flowers. 


Petunia hybrida -.-. no abscission, 
Salpiglossis sinuata 
Datura sanguineum .................. .buds and flowers. 
Salpichrora rhomboidea... no abscission. 


Lycium australis --- no abscission. 


....n0 abscission. 


As might be expected, most of these varieties react to laboratory 
air in the same manner that they do to illuminating gas. In the case 
of laboratory air a longer time and higher temperature is generally 
required before the reaction occurs. All the species, with the excep- 
tion of those which throw off only young buds, detach most of their 
flowers when left in laboratory air overnight. If a window or two is 
left open, allowing fresh aid to enter and at the same time lowering 
the temperature, no abscission occurs. 

It was found that several of the species recorded above, in which 
no abscission or very little abscission occurred, detached more flowers 
when a larger percentage of gas was used or when subjected to 1.5 
per cent gas for a longer time. Thus, both varieties of Lycopersicum 


1918| Kendall: Abscission of Flowers and Fruits in Solanaceae 399 


esculentum, Iochroma tuberosa, Solanum nigrum, and 8S. verbasci- 
folium, upon subjection to 3 per cent illuminating gas for twenty 
hours, throw off all flowers up to those two or three days past anthesis. 
No abscission occurred, however, in any concentration of gas, in 


# 


Nicotiana Bigelovii, N. quadrivalvis, N. multivalvis, Lycium australis, 
Petunia hybrida, Salpiglossis stinuata, or Salpichrora rhomboidea. 

A peculiar condition exists in Solanum umbelliferwm, which throws 
off buds in the illuminating gas but never under any conditions, 1n- 
cluding temperature or the presence of narcotic vapors, throws off 
flowers in which the corolla has fully opened. <A corresponding con- 
dition seems to exist in Nicotiana Tabacum var. macrophylla, F, H154, 
N. Sanderae, N. rustica var. brasilia, and in one other variety of NV. 
rustica, all of which seldom under any conditions detach fully opened 
flowers, although flowers up to that stage are freely abscissed. Thus 
there seems to be, in certain species and at about the time of the open- 
ing of the corolla, a sudden increase in resistance to the external 
stimulus which is causing abscission. In other species this sudden 
increase in resistance does not take place, abscission commonly occur- 
ring at any stage in the development of the flower or fruit and the 
increase in resistance taking place very gradually. In addition, there 
seems to be an intergradation of forms between those in which the 
increase in resistance takes place suddenly and those in which it takes 
place gradually. 

The next subject to be taken up is a consideration of experiments 
5, 6, 7, 8, and 9 on the induction of abscission in small isolated pieces 
of the pedicel. The main purpose of devising these experiments was to 
throw some light, if possible, on the direct or indirect action of the 
external factor in causing ‘‘ abscission. The pedicel of 
F,, H179 was again chosen as material for the following experiments, 


9 


spontaneous 


Fig. 9 


400 Unwersity of California Publications in Botany [Vou. 5 


largely because of the ease and regularity with which abscission is 
induced in this hybrid by sudden changes in the external environment. 
Experiment 5.—This experiment was devised to discover the effect 
of reducing the volume of material proximal to the separation layer 
on the abscission of flowers of Nicotiana as induced by illuminating 
gas. Two series of flowers were cut as in figure 9. In the last two 
flowers represented on the right the cut was made less than 0.5 mm. 
from the separation layer. These flowers were then rolled in damp 
filter paper and left in 1.5 per cent illuminating gas overnight. After 
fifteen hours, abscission had occurred in all the flowers except the one 
represented on the extreme right in the figure. Abscission had 
occurred in one flower in which the cut had been made less than 
0.5mm. from the separation layer. The control to this experiment 
showed that abscission does not occur for several days in a series of 
flowers cut as in figure 9 and kept under normal conditions. 
Experiment 6.—This experiment was devised to show the effect 
upon abscission of reducing the volume of material distal as well as 
proximal to the separation layer. In this case the flowers were cut off 
at varying distances from the separation layer, making the series 
shown in figure 10. The last two pieces on the right in this series 
were cut less than 0.5 mm. on each side of the separation layer so 
that the total length of the pieces was not much above 1mm. In this 
experiment and in similar ones which follow it was necessary to keep 


bb aes 


Fig. 10 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 401 


the material moist. This was accomplished in various ways, but the 
best method was found to consist in placing the pieces on a long strip 
of filter paper one end of which rested in water. In this experiment 
abscission occurred after ten hours subjection to 1.5 per cent illum- 
inating gas in all except the two pieces represented in the extreme 
right of figure 10. Abscission here took place in several pieces rang- 
ing from 1 mm. to 2mm. in length. A microscopic examination of the 
separation surfaces indicated that the process of abscission corre- 
sponded entirely with normal abscission as it occurs in plants in the 
field. Experiments made in a similar manner upon N. Tabacum 
“Maryland”? and Lycopersicum gave similar results. In the control, 
which consisted in keeping pieces of the pedicel as shown in figure 10 
under normal atmospheric conditions, abscission occurred after about 
twenty hours, evidently as the result of no other stimulus than that 
caused through cutting off the flower by severing the pedicel. The 
reaction in the control, however, is much slower than in the case in 
which the added effect of the illuminating gas is operative, indicating 
that the latter factor, although it here serves merely to hasten the 
abscission process, has an effect of some kind on the tissues at the 
base of the pedicel. 

Following these two experiments, a number of attempts were made 
in the same way to induce abscission in longitudinal free-hand sections 
of the pedicel cut for microscopical examination. It was soon discovered 
that the abscission process could be induced in the separation zone in 
thick longitudinal sections of the pedicel by subjecting them to high 
percentage (5 to 7 per cent) of illuminating gas. Cell separation in 
cross-sections through the separation zone could not be induced by any 
means at hand. The following experiments give more detailed results 
in this connection. 

Experiment 7—In this experiment, median, longitudinal sections 
of varying thickness were cut through the pedicels so that the plane 
of the sections corresponded with the plane formed by both the 
pedicel and the main axis of the inflorescence. These sections were 
subjected to 7 per cent illuminating gas, care being taken to keep them 
moist, but not submerged, throughout the entire experiment. The 
best arrangement was found to be one in which the sections rested in 
a thin film of water on one side but were exposed to the air on the 
other. After several hours in the 7 per cent illuminating gas, abscis- 
sion started in the thicker sections but not in the thinner ones. The 
extent to which abscission proceeded depended upon the thickness of 


402 University of California Publications in Botany [Vou. 5 


the section. Abscission became complete in sections 0.3mm. or more 
in thickness, the separation taking place in such a way that a slight 
bending or pulling motion sufficient to break the tracherz divided the 
section into equal halves. In thinner sections, ranging from 0.3 mm. 
to 0.17 mm., abscission starts in the normal position but does not pro- 
ceed to completion, the extent to which the process takes place depend- 
ing, as has been said, upon the thickness of the section. In sections 
much below 0.17 mm. no signs of abscission appear. Also, if the 
thicker sections are shortened in length to any considerable extent by 
cutting off portions of the tissues from either side of the separation 
layer, abscission will not occur. 

The process of abscission as it occurs in these sections corresponds 
exactly to the process in an entire pedicel. Cell separation starts 
independently in the pith and in the cortex, appearing first in that 
part of the cortex corresponding to the ventral region of the pedicel 
where, it will be remembered, abscission starts in the entire flower. 
When mounting the sections on an object slide for microscopical 
examination, the isolated cells in the pith le in position but can be 
easily washed out with a small jet of water. In the cortex a break 
soon appears in the epidermis as the result of manipulation in mount- 
ing and a cavity is formed at that point as the result of the isolated 
cells of the cortex floating out in the water. 

Experiment 7 was repeated in the case of Datura with similar 
results, except that in this case abscission was more active since it 
involved more cells, a situation which one might be led to expect 
because of the differences between the two species in the normal abscis- 
sion of entire flowers. It will be remembered that the separation cells 
of the cortex in Datura are in no way distinguishable from other 
cortical cells; yet even in these sections separation occurs in a definitely 
predetermined position corresponding entirely with the position in 
abscission of the entire flower. It was even noticed that abscission 
started in these sections in the same tissues and in the same manner 
as in normal floral abscission. 

After the thickness of the sections best adapted to obtaining results 
had been determined, the following experiment was performed on 
sections cut from different parts of the pedicel. 

Experiment 8.—In this experiment a series of longitudinal sections 
of the pedicel were cut so that the plane of the sections was at right 
angles to that of the sections cut in Experiment 7. The first section 
was tangential, on the ventral side of the pedicel, and contained only 
the epidermis and a few tiers of cortical cells. Section 2 was also 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 403 


tangential but contained a few trachez on one surface. Section 3 was 
more or less radial, containing two strands of vascular tissue on either 
side. Sections 4 and 5 were similar to sections 1 and 2. On subject- 
ing these sections to illuminating gas it was noticed that abscission 
started first in sections 1, 2, and 3, appearing last in sections 4 and 5. 
This result is exactly parallel with the process as it occurs in normal 
abscission, where the process starts first in the ventral cortex and in 
the pith. 

In passing, mention might be made of the peculiar reaction of the 
tangential sections 2 and 4, which were made up almost entirely of 
cortical cells with a few vascular elements on one side. When abscis- 
sion occurred in these sections, a bending or bowing of the section was 
always noticed. This bending was always such that the tracheal tissue 
was on the concave side, as if the cells of the cortex had undergone 
considerable expansion while the cells of the vascular tissue retained 
their original size. From the work of Richter and others, it may be 
expected that subjection of portions of plant tissues to illuminating 
gas would cause an increase in turgor in the cells concerned. Thus, it 
is probable that the bending of the sections, as described above, is due 
to the increase in turgor of the cortical cells caused by the narcotic 
effect of the illuminating gas. The extent of the bending was such 
that most of the cells in the cortex as well as the separation cells must 
have been involved in the process. On repeating the above experiment 
with Datura, a similar bending of the tangential sections was even 
more pronounced than in Nicotiana. 

Experiment 9—As mentioned above, efforts to induce abscission 
failed in thin sections. The sections in Experiment 9 were cut so that 
they were thin in the separation layer but thick on either side. Both 
surfaces of these sections were thus cut slightly concave so that the 
sections were thickest at the ends and thinnest in the middle, where 
the separation zone was located. The sections were then subjected to 
7 per cent illuminating gas as in Experiment 7. It was not possible 
to cut very thin free-hand sections of the shape described, but it was 
demonstrated without a doubt that abscission occurred in sections of 
this peculiar shape which were thinner in the separation zone than 
those in Experiment 7 where abscission had failed to occur. 

Certain conclusions which can be drawn from experiments 5, 6, 7, 
8, and 9 are given below. 

1. Abscission can be induced by allowing the external factor to act 
directly upon the cells in the vicinity of the separation zone (Expts. 
6, 7, and 8). 


404 University of California Publications in Botany [ Vou. 5 


2. Abscission induced by the above methods in isolated pieces must 
be independent of transportation of material from the rest of the plant. 

3. The fact that abscission cannot be induced in thick cross-sections 
of the separation zone shows that cell separation cannot be induced 
by the action of the external factor directly on the separation cells. 

4. It is necessary that a certain proportion of the tissues of the 
pedicel be in intercellular connection with the cells of the separation 
zone before cell separation will occur, but this proportion is surpris- 
ingly small (Expts. 7, 8, and 9). 

5. There is evidently increase in turgor in all the cortical cells of 
the pedicel during abscission induced by the above method (Expt. 8). 


2. AcTION OF ACIDS ON THE SEPARATION CELLS OF Nicotiana 


Under this heading a description will be given of the effect. of 
mineral acids on small isolated pieces such as were used in experiments 
6, 7, 8, and 9. It was stated above (page 364) that by the use of two 
mineral acids together with several stains, no chemical difference could 
be detected between the cell walls of the separation cells and those of 
normal cortical cells. The present work represents an attempt to 
determine, by experimental means and by watching through the micro- 
seope the action of acids on cell walls, whether the cell membranes 
of the separation cells are more subject to hydrolysis than those of 
normal cortical cells. 

Experiment 10.—Small pieces of the pedicel were prepared as in 
figure 10. These pieces were boiled for one or two minutes in 4 per 
cent hydrochloric acid and then washed in water. Upon examination 
it was found that the pieces could be separated into halves through 
the separation zone by a sight pulling or bending motion. Microscopie 
examination of the separation surfaces showed that the break through 
the cells of the separation zone had taken place along the plane of the 
middle lamellae of their walls. This same type of separation was 
brought about without boiling when 10 per cent nitric or hydrochloric 
acid was allowed to act on the pedicels for approximately five minutes. 
When longitudinal sections are used in place of entire pedicles, the 
same results are obtained but much more rapidly. It was also noticed 
that separation under these latter conditions takes place more quickly 
in younger pedicels than in older ones. In the pedicels of fully 
developed fruits no separation could be induced, but in those of 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 405 


immature fruits separation occurred in the cortex but failed to take 
place within the vascular cylinder. 

Experiment 10 at first glance would seem to indicate that the cell 
walls of the separation cells are more subject to hydrolysis than normal 
cortical cells. Another interpretation is possible, however. Actual 
separation which takes place through the separation zone may be due 
to the fact that the cells in this zone are small and have a tendency 
to be isodiametric, whereas the remaining cells of the cortex are larger 
and are elongated parallel to the long axis of the pedicel. Hydrolysis 
of the cell walls may go on with equal rapidity in all the cortical cells 
at the base of the pedicel, yet upon bending or pulling separation may 
take place through the region of isodiametric cells because of the inter- 
locking of the elongated cells in the rest of the cortex. An attempt 
was made to gain further evidence on this point by observing through 
the microscope the action of acids on the cell walls of the tissues con- 
cerned. When the action of the acids is thus observed, the walls are 
seen to soften and to swell to two or three times their normal thick- 
ness. This effect is all the more noticeable if the walls initially are 
comparatively thick. Now, since the cells of the separation zone are 
small and somewhat collenchymatous, or at least have thicker walls 
than normal cortical cells, the process of swelling in the cell wall is 
most conspicuous in that region. Indeed, hardly any swelling can be 
perceived as a result of the acid treatment in the cell walls of normal 
parenchyma cells of the cortex. However, when a form such as 
Lycopersicum is examined in which there is a distinct layer of col- 
lenchyma beneath the epidermis for the entire length of the pedicel, 
this collenchyma appears to be affected at the same time and in the 
same manner as the cells of the separation zone of Nicotiana. Also 
in Nicotiana there seems to be a certain amount of similarity in 
reaction to acids between the smaller cells of the cortex just beneath 
the epidermis and those of the separation zone. The conclusion can 
thus be drawn that the cell walls of the separation cells are no more 
readily hydrolyzed than those of normal collenchymatous tissues. Of 
course, the fact still remains that the collenchyma of the cortex may 
be more subject to hydrolysis than the cortical parenchyma. Now 
the small cells of the separation zone not only extend across the base 
of the pedicel but also spread throughout the general region at the 
base of that organ; it was therefore noticed that the swelling of cell 
walls was by no means confined to cells of the separation layer but 
was more or less prominent throughout the whole general region at 
the base of the pedicel. 


406 Unwersity of California Publications in Botany [ Von. 5 


The general results of these observations are in a sense negative 
and seem to indicate that the walls of the separation cells are no more 
subject to hydrolysis than the walls on either side. This, of course, 
does not preclude the possibility that a difference exists which is too 
shght to be detected. It appears, however, that the general region 
at the base of the pedicel may be more subject to hydrolysis than the 
more distant portions. 


3. INDUCTION BY MECHANICAL INJURY 


The results of experiments on the induction of abscission by mechan- 
ical injury are recorded in tables 2, 3, 4, and 5, which have already 
been considered under the heading, ‘‘Time of Abscission’’ (page 384). 

Several facts of interest brought out by table 2, which deals with 
Nicotiana Langsdorffii var. grandiflora, are summarized below. 

1. It appears that removal of or injury to the capsule does not 
cause abscission in mature fruits (table 2, a, b, and h; table 3, c and 
d). The same types of injury generally do cause abscission in im- 
mature fruits. 

2. It seems that a transverse cut completely through the flower at 
the distal end of the calyx causes abscission only in buds or flowers 
near anthesis (table 2, c). It appears, however, that such a cut 
proximal to the distal end of the calyx causes abscission in flowers 
several days past anthesis as well as in buds (table 2, a, b). 

3. Removal of the entire calyx causes fall in very young buds only 
(table 2, d). 

4. It seems that slitting both the corolla and calyx longitudinally 
on both sides from tip to base does not induce abscission even in young 
buds (table 2, e). 

5. Entire removal of the style or stamens causes fall only in young 
buds (table 2, f and g). 

6. It appears that injuries to the pedicel do not cause abscission, 
provided the flower is not entirely cut away (table 2,7). Just here it 
is worth mentioning that two of the pedicels cut transversely as 
recorded in table 2, 7, were cut so deep that the flowers bent over 
and hung only by a few vascular strands and cortical cells. The 
wound healed over, however, and the two flowers matured with the rest. 

7. It is evident that injuries which reach the ovary are much more 
effective in causing abscission than injuries affecting the other parts 
of the flower (table 2, b and e). 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 407 


8. Fertilization has no influence whatever in preventing abscission 
when the latter is induced by a transverse cut completely through the 
flower at the base or middle of the calyx (table 3, ¢ and d). 

9. Certain types of injury, such as entire removal of the calyx and 
stamens or removal of the entire calyx and half the corolla, evidently 
cause abscission only by preventing fertilization (table 3, a and b). 

Taking up now the results given in table 4, which dealt with 
HF. H179, it will be seen that this hybrid is more sensitive to injury 
than is NV. Langsdorffii. Nevertheless, it is very plain that the general 
conclusions announced above for this latter species hold for F, H179 
also. There follows a partial summary of the results in table 4 and 
a comparison of these results with those obtained in the experiments 
on N. Langsdorffit. 

1. It seems that removal of the calyx causes fall of much larger 
buds than in N. Langsdorffii (table 4, d). 

2. F, H179 is evidently much more sensitive in its abscission re- 
action to a transverse cut through the flower at the middle of the calyx 
than N. Langsdorffir (table 4, a). 

3. It would seem that slitting the calyx and corolla even to the 
extent of dividing these organs into four longitudinal strips does not, 
as a general rule, cause abscission. Such an injury does cause abscis- 
sion only in extremely small buds (table 4, g). 

4. It appears that puncturing the calyx, corolla and ovary so 
that a hole is formed about 2 mm. in diameter in the latter organ 
causes fall in flowers of all sizes up to two or three days past anthesis 
(table 4, h). Since it is evident that such a hole through the calyx 
and corolla alone would not cause abscission (table 4, g), abscission 
in this case must be induced by injury to the ovary. 

5. It is evident that a slit completely through the pedicel for its 
entire length fails to cause fall in buds or open flowers, but where an 
effort is made to destroy completely the connection between the flower 
and stem abscission will occur (table 4, 7). 

6. Removal of the style or stamens, as a general rule, causes fall 
only in young buds, but removal of the former organ is probably more 
effective in causing flower-fall than removal of the stamens (table 4, e 
and f). On the other hand, where half the corolla is removed along 
with the stamens fall occurs in larger buds than where only the latter 
organs are removed (table 4, b). 

7. Removal of only half the corolla apparently does not induce 
abscission (table 4, c). 


408 University of California Publications in Botany [Vou. 5 


8. Mature capsules of F, H179 are apparently more sensitive to 
injury than those of NV. Langsdorffii (table 4, 7). 

The table dealing with the experiments on Lycopersicum indicates 
that flowers of this genus are remarkably resistant to injury, fall 
occurring only as the result of stimulation when the ovary is injured 
(table 5, c and d). Since a large number of tomato flowers are nor- 
mally abscissed from the different inflorescences on a plant, the sev- 
eral exceptions to the above statement noted in the table probably 
demonstrate to what extent the normal physiological condition of the 
plant affects the matter. It seems to be the opinion of most gardeners 
who are familiar with the tomato plant that floral abscission in this 
species is more dependent upon soil conditions than upon injury, or 
sudden changes in climatic conditions. It would seem, however, that 
injuries to very young fruits normally cause fall, but in this case a 
stage of development is soon reached at which injury to the berry has 
no effect in inducing abscission (table 5, f). 

Taking the general results of all the experiments into consideration, 
it is seen, in the first place, that where injury of a certain type causes 
fall, a stage of development of the flower is soon reached beyond which 
the injury no longer causes fall. The increase in resistance to the 
stimulus of mechanical injury takes place gradually in the species 
investigated, but some of the species are much more resistant than 
others. In the second place, injuries to the ovary generally cause 
flower-fall. Thirdly, whether or not flower-fall occurs as a result of 
injury to other flower parts depends in some way upon the quantity 
of material removed. Fourthly, injury to the pedicel does not cause 
abscission unless it breaks entirely the cellular connection between 
flower and stem. Lastly, it is improbable that fall induced by injury 
is due to checking the transpiration stream, since injury to the ovary 
could have no such effect. Also, a cut across the pedicel so that the 
flower hangs by only a few trachee must check transpiration from the 
flower considerably, yet in this case no abscission occurs. 

It was suggested by Bequerel that injury might cause abscission 
by checking the transpiration stream which passes up through the 
pedicel. Considerable doubt has already been cast on this point in the 
above discussion. In order to throw more light on this question the 
following experiment was performed in an effort to determine whether 
checking the transpiration stream of itself and unaccompanied by 
mechanical injury would cause abscission. 

Experiment 12.—As a means of checking transpiration from the 
flower a coating of paraffin seemed desirable because it hardens 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 409 


quickly, thus permitting several coats to be applied. It was doubtful 
whether other substances, such as lard, cocoa butter or vaseline, which 
might have been used, would not have been prevented from completely 
covering the flower in one coating by the presence of numerous hairs 
and glandular fluid on the calyx. In this experiment flowers were 
immersed in melted paraffin to within a millimeter of the separation 
zone and allowed to stand in water under normal atmospheric condi- 
tions. As a test for abscission, the shoot was shaken or individual 
flowers tapped from time to time. It was found that several Nicotiana 
varieties and hybrids differed in their reaction to this treatment as 
they did in their reaction to illuminating gas. In NV. Tabacum ‘‘Mary- 
land,’’ for example, paraffining the flowers failed to cause abscission 
for six days, at the end of which time the flowers began to fall, as did 
those of the control. Some varieties, however, under such treatment, 
throw off buds at the end of twenty-four hours, but open flowers of 
the same varieties are never shed. Whether or not the buds fell in 
these varieties depended largely on the temperature, at lower tempera- 
tures no fall occurring. Also, in cases where abscission of buds did 
occur it was evident that something was actually impeding the pro- 
cess; none of the white substance formed by the isolated cells was seen 
at the base of the pedicel and the buds had to be shaken or tapped 
quite severely before they fell. 

The results of Experiment 12 and the various observations on the 
induction of abscission by mechanical injury render it extremely 
unlikely that checking the transpiration stream is ever a direct cause 
of abscission. The few cases recorded above in which such a condition 
seems to cause abscission can be better explained by the action of some 
other factor than that of interference with transpiration. 

In connection with these experiments upon the effect of checking 
transpiration the results of Lloyd and Balls on the effect of root 
pruning, ete., in cotton must be mentioned. It was found that a pre- 
mature shedding of flowers and young bolls followed root pruning 
and further that, in general, there is a relation between boll-shedding 
and the rise and fall of the water-table. Proof positive is not sup- 
plied that root pruning causes fall of flowers by reducing the water 
supply of the plant body, and any number of other factors may enter 
in after such mutilation to bring about, in part at least, such a result. 
Experiments reported in the present paper seem to leave no doubt 
that, in Nicotiana at least, temperature is a more important factor in 
controlling abscission than water supply. 


o 


410 University of California Publications in Botany [ Vou. ¢ 


4. THe Apinity oF CERTAIN SPECIES TO THROW OFF PEDICELS 
FROM WHICH ALL THE FLORAL ORGANS HAvE BEEN 
REMOVED, AS RELATED TO THE INDUCTION OF 


ABSCISSION BY MmrcHANICAL INJURY 


It was soon noticed in the experiments that all plants of a species 
in which floral abscission occurs throw off the remains of the pedicel 
when this organ is severed at any point distal to the separation layer. 
If after such an operation no abscission occurs, it can be safely con- 
eluded that floral abscission never occurs in that species. Petunia 
hybrida, Salpiglossis sinuwata, Salpichrora rhomboidea, and Lyciwm 
australis are the only species of the list in table 6 which do not absciss 
flowerless pedicels in this way. Nicotiana Bigelovii, N. quadrivalvis, 
and N. multivalvis occasionally do not throw off pedicels under such 
conditions. The reaction time in cases where the last three species do 
absciss severed pedicels is very slow (four to fourteen days). 

Turning now to the relation of these observations to the induction 
of abscission by mechanical injury, it is first necessary to recall the 
controls used in Experiments 5 and 6 (ef. pages 399 and 400). <A fur- 
ther consideration of the reaction of these controls will suggest that 
mechanical injury can induce abscission by the action of the stimulus 
directly on the cells in the vicinity of the separation zone. The con- 
trol used in Experiment 5, it will be remembered, showed that abscis- 
sion does not occur under normal conditions in a series of flowers cut 
as in figure 9. From the control used in Experiment 6 it is evident 
that merely cutting off the flower at varying distances from the sep- 
aration layer, forming pieces as represented in figure 10, causes ab- 
scission to oceur, evidently as the result of no other stimulus than 
that of severing the pedicel. Now, if the eut be made through the 
pedicel at a point approximately 1 mm. distal to the separation layer 
in flowers, as represented on the extreme right of figure 9, abscission 
will occur in the remaining piece, which is now scarcely 2 mm. in 
length. It is evident that the stimulus caused by severing the pedicel 
must act directly on the cells in close proximity to the separation 
zone. Practically the same results are obtained when the transverse 
cut is made through the base or middle of the calyx. There is no 
reason to suppose that the stimulus set up by cutting through the 
flower near the base or middle of the calyx differs in any fashion from 
that offered by a cut severing only the pedicel. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 411 


Several interesting conclusions are brought out by an examination 
of the above facts. In the first place, the abscission of the remains of 
severed pedicels is probably imdependent of the transportation of 
materials from the rest of the plant to the separation zone. It may 
result from the action of the stimulus directly on the cells in the 
vicinity of the separation layer and is, therefore, largely independent 
of such physiological processes as transpiration which might concelv- 
ably enter in. In the second place, abscission induced by mechanical 
injury is probably of the same nature as that of severed pedicels and 
therefore probably results from the action of the stimulus directly on 
the cells in immediate proximity to the separation layer. 


SUMMARY 


The final summary of results given below is presented under 
several headings corresponding to those of the main body of the 
paper. Unless otherwise stated, the results given may be taken as 
applying to all the species of the Solanaceae in which abscission was 
found to occur. First is presented a complete list of the species which 
were investigated, indicating by 1 those in which floral abscission 
never occurs, by 2 those in which it very seldom occurs, and by 3 
those which were actually examined microscopically to determine the 
histological structure of the separation zone and the method of 
abscission. 


3 N. Tabacum var. macrophylla 3 Solanum umbelliferum 
3 N. sylvestris S. tuberosum 
3 N. Tabacum ‘‘ Maryland’’ S. jasminioides 
3 F,\H154 (N. sylvestris X N. Tab. 3 8. verbascifolium 
var. macrophylla) S. nigrum 
3 F,H179 (N. sylvestris X N. Ta- 2,3 Iochroma tuberosa 
baeum ‘‘Cuba’’) 3 Cestrum fasciculatum 
3 F,H36 (N. sylvestris X N. Tab. var. Lycopersicum esculentum var. vul- 
angustifolia) gare 
N. glauca 3 L. esculentum var. pyriforme 
3 N. rustica (2 varieties—not bra- 1,3 Petunia hybrida 
silia) 1,3 Salpiglossis sinuata 
2,3 N. Bigelovii (3 varieties) 3 Datura sanguineum 
2 N. quadrivalvis (2 varieties) 1 Salpichrora rhomboidea 
2 N. multivalvis 1 Lycium australis 


N. Sanderae 
N. rustica var. brasilia 
N. suaveolens 


412 Umversity of California Publications in Botany [ Vou. 5 


HistoLoGy AND CYTOLOGY OF THE PEDICEL 


1. The separation layer arises in all the species listed above, except 
Lycopersicum and Solanum tuberosum, at or near the base of the 
pedicel. In the latter two species the layer is located near the middle 
of the pedicel, but even in these cases, if one considers the pedicel to 
be composed of two internodes, the layer occurs at the base of the 
most distal internode. 

2. The separation layer is preformed, ready to function at any 
stage in the development of the flower and represents (cf. Kubart’s 
first type, page 350) a portion of the primary meristem which has 
retained some of its originally active condition. 

3. In all the species except Datura the separation cells are char- 
acterized by their small size, isodiametric shape, large amount of 
protoplasm and somewhat collenchymatous appearance. A study of 
the early histological development of the pedicel indicates that the 
small size of the separation cells does not necessarily bear any relation 
to abscission. This statement is supported by the fact that in Datura 
there is absolutely no visible difference between the separation cells 
and any other cells of the pedicel. 

4. Various tests with stains, acids, and alkalis fail to indicate any 
chemical difference between the cell walls of the separation cells and 
the walls of neighboring cortical cells which do not separate. How- 
ever, the middle lamellae of cell walls in the general region at the 
base of the pedicel seem somewhat more easily hydrolysed by acids 
than in the more distal portions. 

5. A study of the early histological development of the pedicel in 
Nicotiana and Lycopersicum shows that the grooves near which the 
separation zone arises do not necessarily bear any relation to abscis- 
sion. The grooves are formed because, in the development of the 
pedicel, certain cells do not increase in size so fast as the neighboring 
cells on either the proximal or distal side. 

6. The development of mechanical tissue in the pedicel of Nicotiana 
continues through the separation layer, thus frequently holding the 
fruit on the plant in spite of the fact that abscission commonly oceurs 
in the cortex. In most of the berry-forming species of the Solanaceae 
this mechanical tissue does not become continuous through the separa- 
tion layer and thus offers no impediment to fall when abscission oceurs 
in that region. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 413 


Tue PRocEss oF ABSCISSION 


1. The process of abscission conforms to the usual type, which 
involves the separation of cells along the plane of the middle lamella 
of the cell wall separating them. 

2. No cell divisions or elongations were observed to accompany 
abscission. 

3. All the cells across the pedicel in the region of the separation 
layer take part in separation except the trachew and cuticle, which 
must be broken mechanically. The total number of cells which may 
be involved is greater in some species than in others. This number 
may also vary in the same species because of changes in the external 
conditions. 

4. Cell separation is brought about by the hydrolysis and conse- 
quent dissolution of the middle lamella (primary cell membrane) or 
perhaps both the primary and, in part, secondary cell membranes. 
The ageney active in the hydrolysis of the cell membranes is probably 
an enzyme. 

5. An increase in cell turgor frequently occurs during abscission, 
but probably serves merely to hasten and facilitate the process. Most 
of the frequently observed expansion and the turgid appearance of 
the separation cells during abscission are probably due to the natural 
release of pressure caused by the dissolution of the middle lamellae. 

6: Abscission of the style and corolla in Nicotiana and Datura 


‘ 


resembles, to a large extent, abscission of the flower. 


TIME OF ABSCISSION 


1. The length of time between anthesis and normal flower-fall due 
to lack of fertilization differs among the varieties of Nicotiana. This 
variation was found to range between an average of five to eighteen 
days in some fifteen species and varieties of Nicotiana. A much 
smaller range of variation (0.7 to four days, with the largest fre- 
quency in the three day group) was noted for the time between an- 
thesis and fall of the corolla after pollination. 

2. The stimulation of the stylar tissues by the growth of the pollen 
tubes tends to shorten the time between anthesis and fall of the 
corolla, this effect being independent of fertilization. Such stimula- 
tion of the stylar tissues has no appreciable effect upon floral ab- 
scission. 

3. Floral abscission occurs in F, H179 seven hours after subjecting 

“shoots of the plant to1.5 per cent illuminating gas at a temperature 


414 University of California Publications in Botany [ Vou. 5 


as 


of 19° C. It occurs in Nicotiana Tabacum ‘‘Maryland’’ in eight hours 
under the same conditions. The actual time involved in the process of 
cell separation in the above-mentioned cases lies within thirty to forty 
minutes in the hybrid and within forty-five to sixty minutes in the 
Tabacum variety. Normal abscission in these forms is much slower 

4. The length of the reaction time in cases of flower-fall due to 
mechanical injury shows that this length of time depends more on 
the age of the flower than on the type of injury. 

5. Temperature is the most important conditioning factor in esti- 
mates of the time of abscission. 


EXPERIMENTAL INDUCTION OF ABSCISSION 


1. Floral abscission is induced, in a large number of the species 
investigated, by illuminating gas or laboratory air. The increase in 
resistance to abscission stimulated in this manner takes place suddenly 
in some species, since abscission will not occur after the opening of 
the corolla. In other species this condition does not exist. 

2. It is possible to induce the process of abscission with illuminat- 
ing gas in small isolated pieces of the pedicels or in longitudinal sec- 
tions of the pedicel cut free-hand from fresh material. 

3. Abscission in Nicotiana and Lycopersicum is induced by certain 
types of severe injury and not by others. Injury to the ovary seems 
more effective in causing abscission than injury to other parts of the 
fiower. In the case of these other flower parts, it seems necessary that 
a certain amount of tissue be actually removed or destroyed before 
fall occurs. Injury to the pedicel does not cause abscission unless it 
breaks entirely the connection between floral organs and stem. 
Flower-fall in Lycopersicum is not readily induced by injury. Floral 
abscission in this genus is more dependent upon physiological condi- 
tions brought on by abnormal soil conditions. 

4. Experiments on the induction of abscission in small isolated 
pieces and in flowers with only a small portion of the stem proximal 
to the separation layer attached indicate that the stimulus produced 
by the action of external factors such as illuminating gas and mechan- 
ical injury can eause abscission by acting directly on the cells in close 
proximity to the separation zone. The action of external factors is 
thus largely independent of such physiological processes as transpira- 
tion which might enter in. This statement is supported by experi- 
ments which show that abscission is not necessarily induced by 
checking transpiration from the flower. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 415 


CONCLUSION 


It is proposed in what follows to take up consideration of such 
phenomena in connection with abscission as are still but slightly 
understood. One of the most perplexing of these is undoubtedly the 
definitely predetermined location of the separation layer when no 
morphological and sometimes no physiological (Datura) difference can 
be detected between the cells that separate and those that do not. 
There need be no doubt, however, that such a difference does exist 
and that a sufficient refinement of technique will serve to detect it. 

In considering this matter further it may be recalled that the 
separation layer in axial abscission is located at or near the base of an 
internode. There is undoubtedly some connection between this fact 
and the fact that the cells most active physiologically are often found 
in this region. The growth of an internode may be brought about by 
the action of an intercalary meristem located at the base of the organ 
and a meristem so located in some eases retains its original activity 
in the mature internode. Now it is well known that the walls of young 
active cells are more readily subject to hydrolysis than the walls of 
older cells, because of the fact that the former contain more water. 
If we assume, then, that the internode is a metabolic gradient with 
the most active cells at the base, it would be expected that the walls 
of these cells would be more subject to hydrolysis than any other cells 
of the internode. If some hydrolysing agency becomes active 
throughout the pedicel, it might be expected that the walls of the 
cells at the base of the internode would react first, causing their sep- 
aration and thus cutting off the flower or internode. By assuming 
in this way that separation always takes place through the most 
active cells of the internode it seems possible to explain the predeter- 
mined location of the separation layer. 

There is undoubtedly some connection between the above problem 
and the fact that some plants must perfect a separation layer before 
detachment can take place. In such eases the tissues at the base of 
the organ are too old for separation. The same stimulus which causes 
abscission in some species causes a renewal of activity at the basal 
region of an organ, resulting in cell divisions and new cells. These 
new cells may, under a continuation of the stimulus, separate one 
from another. 

Another perplexing problem, which also includes many subsidiary 
problems, relates to the exact course taken by the stimuli in causing 


416 University of California Publications in Botany [Vou. 5 


abscission. Experiments described in the present paper have indi- 
cated that this course may be direct as well as indirect. Assuming for 
the present that some of the factors bringing about abscission always 
act directly while others act indirectly, we might classify the general 
factors operative in the case of the Solanaceae as follows: 


DIRECT . Narcotie vapors. 


1 

2. Injury to floral organs. 

3. Sudden rise in temperature. 
4. Lack of fertilization. 
5 
6 


Inprrect 5. Changes in soil conditions. 


. Factors evident in normal physiological development. 


The direct factors act directly on the cells at the base of the pedicel 
and consequently the reaction time must be comparatively rapid. The 
indirect factors act indirectly through the general physiological con- 
dition, which in turn furnishes the direct stimulus for cell separation. 
In the latter case the reaction time must, as a general rule, be slow. 
The nature of factors under 6 are most difficult to understand. An 
example of the action of these factors would be given in those cases 
where most of the flowers of an inflorescence are normally abscissed 
leaving only one or two to continue development, and in those species 
which absciss male flowers after anthesis. 

A further analysis of the course of the abscission reaction intro- 
duces another unsettled problem—the nature of the agency which is 
directly responsible for the dissolution of the middle lamella. It has 
been pointed out before that an enzymatic body of some kind is prob- 
ably involved. The following discussion brings out certain facts 
which it is necessary to take into consideration when speculating as 
to the nature of this supposed enzyme. The activity of the enzymatic 
body must be subject to both internal and external conditions. The 
enzymatic material must also be extremely sensitive to slight changes 
in the normal environment. It must be continually present in the 
cells of the separation zone and ready at any moment to react to such 
changes in the environment. A+ comparison of several species in 
regard to their abscission reactions to the factors listed above indicates 
that this supposed enzyme must be more sensitive in some species 
than in others. Indeed, in certain species in which no abscission 
oceurs the enzyme must be absent from the region of the separation 
zone or entirely inactive. Finally, it seems necessary to assume that 
in certain species the action of the enzyme is suddenly inhibited at 
about the time of the opening of the corolla. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 417 


It has been noticed in all the experiments detailed above that 
older flowers are less subject to ‘‘spontaneous’’ 
younger ones. The transition line as to size or age beyond which 


abseission than 


no abscission occurs can not in most eases be definitely drawn; that 
is to say, the development of a resistance to stimuli takes place grad- 
ually. This is probably explained by the fact that cell walls gradually 
become less subject to hydrolysis with age. The celluloses and pec- 
toses lose water with age and it is well known that these compounds are 
subject to hydrolysis in proportion to the amount of water they con- 
tain. In those cases where the increase in resistance to stimuli takes 
place suddenly it is necessary, as suggested above, to assume some 
kind of inhibitor of the enzymatic action. 

The effect that pollination has in hastening abscission of the 
corolla is a subject which is related to the phenomena described by 
Fitting (1909) for orchids. The phenomena are as yet only slightly 
understood. The explanation seems to involve some relaying of 
stimulus from cell to cell. This is also involved in the explanation of 
floral abscission induced by injury to the ovary. These two cases 
and others indicate that in some instances, at least, abscission responses 
are related to tropistiec responses as Fitting (1911) has suggested. 

Finally, attention may be called to the fact that the most pressing 
need in connection with all the problems mentioned above is, in the 
first place, to establish by some experimental means a definite connec- 
tion between some enzymatic body and the process of abscission and, 
in the second place, more definite knowledge as to the role which cell 
turgor plays in cell separation. Taking all the facts into considera- 
tion, it is evident that abscission is fundamentally a physiological 
problem, the crux of which lies, as in all such problems, in the bio- 
chemistry of the cell. 

The studies reported upon above were carried on under the direc- 
tion and supervision of Professor T. H. Goodspeed and I am under 
deep obligation to Professor F. E. Lloyd for many valuable sugges- 
tions both throughout the course of the experiments and during the 
preparation of this report of them. 


418 Umversity of California Publications in Botany [ Vou. 5 


2 


LITERATURE CITED 


ATKINS, W. R. 
1916. Some recent researches in plant physiology, p. 64. 


Batis, W. 
1911. Cotton investigations in Egypt, 1909-1910. Cairo Sci. Jour., vol. 5, 
p. 221. 
BECQUEREL, W. 
1907. Sur un eas remarquable de autotomie de pedonele floral de tabac 
provoqué par le traumatism de la corolla. C.—R. Acad. Sci. Paris, 
vol. 245, p. 936. 


Brown, H. T., and Escoms, F. 
1902. The influence of varying amounts of carbon dioxide in the air on 
photosynthetic process of leaves and the mode of growth. Proc. 
Roy. Soe. London, vol. 70, p. 97. 
CoRRENS, C. 
1899. Vermehrung der Laubmoose. Jena, 1899. Quoted from Lloyd (1914a). 
East, E. M. 
1915. Phenomenon of self-sterility. Am. Nat., vol. 49, p. 77. 


Firrine, H. 
1909. Die Beinflussung der Ochideenbluten durch die Bestaubung und dureh 
andere Umstande. Zeitschr. Bot., vol. 1, p. 1. 


1911. Untersuchung iiber die vorzeitige Entblatterung von Bliiten, Jahrb. 
wiss. Bot., vol. 49, p. 187. 


GoopsPEED, T. H., and KENDALL, J. N. 
1916. An account of the mode of floral abscission in the F, species hybrids 
of Nicotiana. Uniy. Calif. Publ. Bot., vol. 5, no. 10, p. 293. 


GorTNER, R. A., and Harris, J. A. 
1914. On axial abscission of Impatiens Sultani as the result of traumatic 
stimuli. Am. Jour. Bot., vol. 1, p. 48. 


Hannice, E. 
1913. Untersuchung tiber das Abstossen von Bliiten us.w., Zeitschr. Bot., 
vol. 5, p. 417. 


HOEHNEL, F. R. 
1878. Ueber den Ablésungvorgang der Zweige einiger Holzegewachse und 
seine antomischen Ursachen. Mitteil. forstl. Versuch. Oester., vol. 1, 
no. 3; vol. 3, no. 2. 
KuBaArtT, B. 
1906. Die organische Ablésung der Korollen nebst Bemerkung tiber die 
Molsche Trennungschichte. S.—B. Akad. Wien, Math-nat. Kl., vol. 
115.1, p. 1491. 
Luoyp, F. 
1914a. Abscission in flowers, fruits and leaves. Ottawa Nat., 1914. 
1914b. Injury and abscission in Impatiens Sultani. Quebee Soe. f. protection 
of plants, 1914, p. 72. 
1916a. Abscission in Mirabilis Jalapa. Bot. Gaz., vol. 61, p. 213. 
1916b. Abscission of flower buds and fruits in Gossypium and its relation to 
environmental changes. Trans. Roy. Soc. Canada, vol. 10, p. 55. 


1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 419 


Lez, E. 
1911. Morphology of leaf-fall. Ann. Bot., vol. 25, p. 51. 


LoeEwl, E. 
1907. Blattabléssung und verwandte Erscheinungen. Proce. Akad. Wien, 


Math-nat. K1., vol. 166, p. 983. 


Mont, H. 
1860. Ueber den Ablésungsprozess saftiger Pflanzenorgane. Bot. Zeit., vol. 
18, p. 273. 
REICHE, C. 


1885. Ueber anatomische Verinderungen welche in den perianthkreisen der 
Bliiten wahrend der Entwicklung der Frucht vor sich gehen. Jahrb, 
wiss. Bot., vol. 16, p. 630. 
Ricuter, O. 
1908. Ueber Turgorsteigerung in der Atmospher von Narkotica. Lotos, 
vol. 56, p. 105. 


RicuTer, O., and GRAFE, V. 
1911. Ueber den Einfluss der Narkotika auf die chemische Zusammensetzung 
yon Pflanzen. S.-B. Akad. Wien, Math-nat. K1., vol. 120.1, p. 1187. 


STRASBURGER, E. 
1913. Das botanische Praktikum, p. 349. 
Tison, A. (quoted from Luoyp 1914a). 
1900. Recherches sur la chute des feuilles chez les dicotyledones. Mém. Soe. 
Linn. Normandie, vol. 20, p. 125. Quoted from Lloyd (1914a). 
WIESNER, J. 
1871. Untersuchung iiber die herbstliche Entblitterung der Holzgewachse. 
S.-B. Akad. Wien, Math-nat. Kl., vol. 64, p. 456. 
1905. Ueber Frostlaubfall. Ber. Deutsch. Bot. Ges., vol. 23, p. 49. 


PLATE 49 


Fig. 1. Base of pedicel of Nicotiana bud showing groove, separation zone, 
and process of abscission well under way in dorsal cortex. 


Fig. 2. Portion of cortex in the separation layer of Nicotiana showing the 
bulging of the epidermis, one of the first signs of abscission. 


[ 420 } 


[KENDALL] PLATE 49 


UNIV. CALIF. PUBL. BOT, VOL. 5 


PLATE 50 


Fig. 1. Portion of the base of the pedicel of Nicotiana at a late stage in 
the process of abscission showing the independent origin of the process in the 
pith. 

Fig. 2. Portion of the cortex in the separation layer of Nicotiana showing 
separating cells next to the vascular system. 


[ 422 ] 


[KENDALL] PLATE 50 


UNIV. CALIF, PUBL. BOT. VOL. 5 


Fig. 1 


i 


Wk hey 
+ 


PLATE 51 


Portion of the separation layer of Nicotiana showing cells in the process of 
separation in the upper part of the section. 


[ 424 | 


[KENDALL] PLATE 51 


UNIV. CALIF. PUBL. BOT. VOL. 5 


il inline aie 


Je a 


ct tera 
, orig 
o a: - 


PLATE 52 


Fig. 1. Portion of dorsal cortex near the groove in the pedicel of Nicotiana, 
showing the abscission process well under way. 

Fig. 2. Group of isolated cells washed off from end of a freshly abscissed 
pedicel of Nicotiana. 


Fig. 3. Single isolated cell showing the thinness of the remaining cell 
membrane. 


[ 426 ] 


[KENDALL] PLATE 


UNIV. CALIF. PUBL. BOT. VOL. 5 


PLATE 53 


Fig. 1. Portion of pedicel of Lycopersicum, showing groove and separation 
zone. 

Fig. 2. Portion of cortex of pedicel of Lycopersicum, showing groove and 
abscission process fairly well along; cell separation first takes place between 
only two tiers of cells before spreading to others. 


| 428 ] 


[KENDALL] PLATE 53 


UNIV. CALIF. PUBL. BOT. VOL. 5 


=e 


A 


SA esis ‘ ay: 


? >a Co" 
“TTR aft a” 


‘ 


“ GNIVERSITY ¢ OP OALIFORNIA PUBLIOATIONS— (Continued) 


6: Contributions to ‘the’ Knowledge: of the California Species of Crusta- 
ceous Corallines. It, by Maurice Barstow Nichols. Pp. 349-370; 


plates 10-13. “April, 1909... Seca IR y ORE ig Se ele oe a ee ee se Lie 


» 4. New Chlorophyceae from California, by. Ratnaniot Lyon Gardner. Pp, 
Pek s _ 871-375; -plate14... April, 1909.20. a as 
Rete Plantae Mexicanae Parpusianae, I, Oy T. 8. Brandeges, ies 877- 396, 
Bj eg MARY GE IUG Sete SR oe san sR serene aa SU Se a Se eat a 
: Index, Pp. ‘397-400. : 
vol 4. 1810-1912. see: 
1.. Studies in Ornamental Troes ‘ane Shrubs, by Harvey. Monroe Hail. Pp. 


1-74; plates 1-11; 15 text-figures. -March; 1910 cscs 
me Gracilariophila, a New Parasite on Gracilaria confervoides, by Harriet 
: Ly Wilson. Pp. 75-84; plates 12-13. May,-1910 —.oo ieee 
8. Plantae Mexicanae Purpusianae, I, by Ti 8,.Brandegee. Pp. 85-95. 
Pia TO] as eee ee ie rs eee eA a ee ee 
4, Leuvenia, a New Genus of Flagellates, by N. L. Gardnor. Pp, 97- 106; 
S ‘ roa eC: As sp ee Sica 1s bx Zl Wo aa MRE cl SR San Cree PURI Oe 
Sao 5, The Genus Sphaerosoma, by William Albert Setchell. Pp. 107-120; 
x S wi) 65 ape ks Saas tae Reh Vp eee ph Aen nent een sen gems pace Me A ae 
§. Vafiations in Nucloar Extrusion Among the Fucaceae, by Nathaniel 
Lyon Gardner, Pp. 121-136; plates 16-17. August, 1910 22 


7. The Nature of the Carpostomes inthe Cystocarp of Alnfeldtia gigarty- 
noides, by Ada Sara McFadden. “Pp. 137-142; plate 18. February, 
Sg 0S pera ae WEA Re A a re DE A, RE kt DM Rc ale 2) el ote CREE ee nea 
» 8, On a Colacodasya front Southern California, by Mabel nis McFadden, 
Pp. 143-150; plate 19. February, 191% 2.4.22. en. 
9, ¥rnetification of, Macrocystis, by Edna Juanita Hoffman, Pp. 151- 158; 


a plate 20, February, 190% 22 fc) eo ee 
: 10. Erythrophyllum @elesserioides J. Ag., Ge Wilfred Charles Twigs, Pp. 
159-176; plates 21-24. “March, 1912 2.2 ec 
11, Plantae Mexicanae Purpusianae, Tt by T. 8. Brandegee. Pp. 177-194, 
Bi Gc hegre ty 8 ip pees a eR tee iar Races en cn TO ae SERRA, SR Ree PB pion Rae 
12. New and Noteworthy Californian Plants, 1, hy Harvey Monroe Hall, 
‘ Bp e-19b-208..7 March, POD a ee ae ar eat eck aoe 
; 18. Die Hydrophyllaceen der Sierra Nevada, by August Brand. Pp. 209- 
as = yr Ak ape eas Cy OR BR het 9 RRA RE ac Ow Sec Sin RO se 
a /- 14, Algae Novae et, Minus Cognitae, I, by William Albert Setchell. Pp. 
nk f 229-268; pirates 2BABN Mays VOU ce Rae meee Baer ee A ete 
15. Plantae Mexicanae Purpusianae, IV, by Townshend Stith Brandegee. 
Pp. BESS Tam ay hl OE sais eis os ee a Lae ae 


a - 16, Comparative Development. of the Cystocarps of <Antithamnion and 
Sree : * Prionttts,- by. Lyman Luther Daines.. Pp. 283- 302; plates 82-34. 


Pity MURR EN VOT Se ee Nee he eee Sh Le EL RN Di Se ease tae ae hare nection 
“17%: Fungus Galls on Cystoseiva and Halidrys. by Lulu May Estee. Pp. 305- 
2 SOLOS Pith SD s Sc AECH;. POLS aa wp eae ease sugges oe ce Pa gecge taooroae 
“18. New Fucaceae, by: Nathaniel Lyon Gardner. Pp. 317+ 374; plates. 36- 
> PCOS cca r hb ARO Te ee eh es ke pee Cg ee Sk oS pe ee Re 
19. Plantae. Mexicanae Purpusianae; v, by Townshend Stith Brandegee. 
: ~ Pp. 375-388: June, 1913 2c Poeddi vaeestaie pacts becca Siete ens ay 
ae Index, pp. 389-397, ; 
=-Vol 6.~ 1912-.- 
ake I. Studies im Nicotiana, I, by William Albert Setchell. Pp.: 1-86, . De- 
seks 1 3) eg Pee nee tS RNR Seen Rea Se pees eee Se Ree OI aN 
Ba eae 2. Quantitative Studies of Inheritance in Nicotiana Hybrids, I, by’ Thomas 
iy .. Harper Goodspeed. Pp, 87-168. , December, 1912 02. 
; ~< $ Quantitative Studies of «Inheritance in Nicotiana- Hybrids, H, by 
Thomas Harper -Goodspeed. . Pp. 169-188, January,-1913 2.0220... 


4, On the Partial Sterility of Nicotiana Hybrids made with N. Sylvestris 
as a Parent, by Thomas Harper Goodspeed. Pp.-189-198... March, 


St RO A a ES OTE NN NS ENS SS oan vege ots Rape as Aen 

5. Notes on the Germination of Tobacco Seed, I, by Thomas eae Good- 
SAS DESH, Fe 190282 Me OLS wi a. te eee ee NS 

6.. Quantitative Studies of Inheritance in Nicotiana Hybrids, Tlf, by. 
= Thomas Harper. Goodspeed. Pp. 223-231, “April, 1915 222.02. 
7. Notes on the Germination of Tobacco Seed, ti, by Thomas Harper 
‘Goodspeed. Pp2-233-248. © Fume; 1995: oes. ean eect oe necennee 


zu 8. Parthenogenesis, Parthendcarpy and Phéenospermy in Nicotiana, by 
: Thomas Harper Goodspeed. Pp. 249-272, plate 35, July, 1915_..... 


.10- 


UNIVERSITY OF CALIFORNIA PUBLICATIONS —(Gontinned) 


9. On the Partial Sterility. of Nicotiana ‘Hybrids made with W. sylvestris: « 


“as a Parent. Il, by T. H. a and A.-H. Ayres. Pp. rae 
plate 36. October, 1916 


10. On the Partial Sterility of. Nicotiana Hybrids made with WN. Silecsirs 


as a Parent. III, An Account of the Mode of Floral Abscission in ~ 


the F, Species: Hybrids, by T..H. Goodspeed a J. at Kendall. 
Pp, 293-299, November, 1916) 2.03. 


1i, The Nature of the F,-Species Hybrids between Nicotiana Siplicaee™ 


and Varieties of Nisotiana tabacum, with Special Reference’ to the 
-€onception of Reaction System Contrasts in. Heredity, by T. H. 


Goodspeed and B. E. ‘Clausen. “Pp, 301-346, plates S748. Janu- he 
AT Ys SOL TE ee aA a a eek See NS SES Gi a Sen te eee a 


12. Abscission of Flowers-and Fruits in the Solanaceae, “with Special i 


Reference 'to Nicotiana, by John N, Kendall. Pp. 347-428, 10 text: 
figures, plates 49-53: . March, 1918 -.....2.. Gaiam re oy 


Vol. 6. 1914- 


1. Parasitic Floridede, I, by William: Albert Setchell. Pp. 1- 34, plates Sa 


Defi ADEN, PONS ee ee Sage 


2. Phytomorula regularis, a Syinmettical Protophyte Related to Coelas- 
trum, by Charles Atwood Kefoid. Pp. 35-40, plate 7, “April, 1914, 


8. Variation in Oenothera ovata, by Katherine Layne Brandegee. Pp. 41- - 


BO, plates 8-95. Sume, LOVE oases ceeese sce stenecan obese ntedaeeesnesease 


4, Plantae Mexicanae Purpusianae, VI, by Townshend Stith Brandegee, aE 
Pps oO1-F'7e APO, Oe ea eS ae ee 


5, The Scinaia Assemblage, by William Albert Setchell, Pp. 79-152, 
plates 10-16, October, 1OV4 nce eon aes De sae 


6, Notes on Pacific Coast Algae. I. Pylaielia»Postelsiae, n.sp..8. New 


Type in the. Genus Pylasea; by Carl Skottsberg. Pp. 153-164, plates © 


Ny ES 1 Pe AE 2h plan eR ee eon See eee eee eyo na LURE SDP Cake SRE ET 


7. New and Noteworthy Californian Plants, Ti, by Harvey Monroe Hall. . 
Pp..165-176, plate 20. October, 1915: . acai TRARY Mt were en ea, VTE 


8, Plantae Mexicanae Purpusianae VII, by Townshend Stith Brandegee.. 


Pp. 177-197. ‘October, 1915 one ee ccc cece ee: 
9. Floral Relations Among the Galapagos Islands, by. A. LG. Kroeber. 
Bp 199-290. Miarehe 1916. oo es ee es SS Ra 
* 40, The Comparative Histology of Certain Californian Boletaceae, by... 
Harry S. Yates: Pp. 221-274, plates 21-25. February, 1916 22 
11. A Revision of. the Tuberales of California, by Helen Margaret Gilkey.- 
Pp. 275-356, plates 26-30.. March, 1916 ooo. c cece ec eec cece 


12. Species Novae vel Minus Cognitas, by T. 8. Brandegee. Pp. 367-861. 


AMPS, <TD Ge eC a ROS a aa eee 
13, Plantae Mexicanae Purpusianae, VILL: by Townshend Stith ‘Biandenes: 


Pps SbS°S7S0:" WALCH VOLT aay cabanas atte conecoecarandrdcnn0n¥sp-coccsicgnynsaccsngnoees oat 


14. New Pacific Coast Marine Algae, I, by Nathaniel Lyon ee Pp. ; 
877-416, plates 31-35. “June, 1917 GE ai te ees amen ese ea oad IR ais ’ 


15. An Account of the Mode-of Foliar Abscission in Citrus, by Robert Ww. 


Vol. 7 1916- ~ 
1, Notes on the Californian Species, of Trillium L.- I, A Report of the 


General Results of Field and Garden Studies, 1911-1916, by Thomas ; 


Harper Goodspeed and Robert Percy Brandt. Pp. 1-24, Plates “14, 
"Octane, POTS. ee en gee eee hs ae Soe 


2. Notes on the Californian Species of Trillium L.° ‘EL. The Nature and | — 
Occurrence of Undeveloped-Flowers, by Thomas Harper Goodspeed ~ 


and Robert Percy Brandt.’ Pp. 25-38, plates-5-6. October, 1916 .. 


-$. Notes on the Californian Species of Trillium L. III. Seasonal Changes 
in Trillium Species with Special Reference to the Reproductive Tis- 


sues, by Robert Percy Brandt, “Pp. 39-68, Blater 7-10." December, 


NE ae Oo | Satta ee rca Foumanlps enin een peeasicte Senet pdtioon Gat meas neon peas tained 
4. Notes on the Californian Species:of Trillium L. » IV. Teratological 
Variations of Trillium sessile var. giganteum H. & A., by Thomas 
Harper Goodspeed. Pp. 69-100, plates 11-17. January, 1917 ......_.. 


Hodgson. Pp. 417-428, 3 text figures. Ferns AOTS Sct icteessaewernes ; 


05 


UNIVERSITY OF ILLINOIS-URBANA 


ABSCISSION OF FLOWERS AND FRUITS IN THE 


“HOU 


3 0112 019151262 


